SCWCD Study Guide

Author: swanky(swanky.hsiao@gmail.com)
Reviser: yoshi(yoshijava@hotmail.com)
Last Updated: 2004-08-30
Version: 1.36

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Objectives 1.1~1.6 (約 7 題)

1.1

處理不同類型的 HTTP request(GET、POST、 PUT)

HttpServlet class:
void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, java.io.IOException
void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, java.io.IOException
void doPut(HttpServletRequest req, HttpServletResponse res) throws ServletException, java.io.IOException

Servlet container 會呼叫 Servlet.service(ServletRequest req, ServletResponse res) throws ServletException, java.io.IOException 來處理 request，繼承 GenericServlet 的 servlet 要 override 此 method 來處理 request
HttpServlet.service(HttpServletRequest req, HttpServletResponse res) throws ServletException, java.io.IOException 會把 HTTP request 交給適當的 method 處理，繼承 HttpServlet 的 servlet 不需要 override 此 method 來處理 HTTP request

比較 ServletContext 與 ServletConfig
ServletContext 的內容屬於整個 WebApp，而 ServletConfig 的內容只屬於單一 servlet 實體

1.2

Triggers for GET request

Clicking a hyperlink
Browsing through the browser's address field

註：HTML FORM 預設使用 HTTP GET request

Triggers for POST request

Submitting an HTML FORM, ONLY if its method attribute is POST
ex: <FORM NAME="name" ACTION="/servlet" METHOD="POST">

Tiggers for HEAD request

Clicking a menu option that makes the browser synchronize offline content with the web site

HTTP GET

把資料附加在 URL 後面(query string)，可以 bookmark 起來，大小不限，但有些 browser 可輸入的 URL 有長度限制(255 chars)，只傳送文字資料(text)

HTTP POST

把資料存在 request 裡面(常用在傳送 ID/password、資料量大的檔案)，大小不限，可傳送任何資料(text 與 binary)

1.3

取得 request 的參數(parameter)

ServletRequest interface:
String getParameter(String paramName)
String[] getParameterValues(String param)
Enumeration getParameterNames()

取得 servlet 的初始參數(deployment descriptor 內 <servlet> 的 <init-param> ) (2.2)

ServletConfig interface:
String getInitParameter(String paramName)
Enumeration getInitParameterNames()

取得 HTTP request header 的資訊

HttpServletRequest interface:
String getHeader(String headerName)
Enumeration getHeaders(String name)
Enumeration getHeaderNames()

設定 HTTP response header

HttpServletResponse interface:
void setHeader(String headerName, String value)
void addHeader(String headerName, String value)

設定 response 的 Content-Type

ServletResponse interface:
void setContentType(String type)

You need to set the content type of the response using the ServletResponse.setContentType() method before calling the ServletResponse.getOutputStream()

取得 response 的 text stream 傳送字元資料

ServletResponse interface:
PrintWriter getWriter() 註：java.io.Writer
└ java.io.PrintWriter、javax.servlet.http.JspWriter (8.6、12.7)

取得 response 的 binary stream 傳送任何資料

ServletResponse interface:
ServletOutputStream getOutputStream()
註：java.io.OutputStream
└ javax.servlet.ServletOutputStream
註：若在 JSP 中呼叫 response.getOutputStream()，會有 IllegalStateException

將 HTTP request 重新導向別的 URL (5.3 URL-rewriting)

HttpServletResponse interface:
void sendRedirect(String newURL)

1.4

Servlet scopes：Request(ServletRequest)、Session(HttpSession)、Context(ServletContext) (10.2 JSP scopes)

在 ServletRequest、HttpSession、ServletContext 取得、設定、清除 attribute (5.1 session attribute)

Object getAttribute(String name)
void setAttribute(String name, Object value)
Enumeration getAttributeNames()
void removeAttribute(String name)

獨立存取資源

ServletContext interface:
java.net.URL getResource(String path)
java.io.InputStream getResourceAsStream(String path)
註：可存放資料的地方

Servlet Context (on server)	同一 WebApp、不同 user、不同 session，網頁間可共享的 memory
Session (on server)	同一 WebApp、同一 user、同一 session，網頁間可共享的 memory
Request	同一 request
Cookies (on client)	同一 WebApp、同一 user、不同 session，網頁間可共享的 file

1.5

Servlet life-cycle (8.5 JSP life-cycle)

Servlet interface:
void init(ServletConfig config) throws ServletException：每個 servlet object 只呼叫一次
void service(ServletRequest req, ServletResponse res) throws ServletException, java.io.IOException
void destroy()：只呼叫一次，但若是 servlet container 當掉，可能不會呼叫到
註：同一 servlet class 可有多個 servlet object：Single-Threaded 或在 deployment descriptor 設定不同的初始值

1.6

取得 RequestDispatcher 物件

ServletRequest interface:
RequestDispatcher getRequestDispatcher(String path)：path 可以是相對路徑或絕對路徑(以"/"開頭的路徑)
ServletContext interface:
RequestDispatcher getRequestDispatcher(String path)：path 只能是絕對路徑(以 "/"開頭的路徑)
RequestDispatcher getNamedDispatcher(String name)：name只能是 Servlet 的資源名稱

使用 RequestDispatcher 來 include、forward 到另一個 web resource (4.2 forward 到錯誤網頁)

RequestDispatcher interface:
void forward(ServletRequest req, ServletResponse res) throws ServletException, java.io.IOException
void include(ServletRequest req, ServletResponse res) throws ServletException, java.io.IOException

比較 RequestDispatcher.forward 與 HttpServletResponse.sendRedirect (1.3、5.3)

RequestDispatcher.forward：transparent to the browser，request 是同一個
HttpServletResponse.sendRedirect：新的 request

Objectives 2.1~2.2 (約 4 題)

2.1

網站目錄結構

WebAppName

├ 所有 html, jsp, 圖片等等檔案
├ WEB-INF

├ web.xml(WebApp deployment descriptor)
├ classes(WebApp 類別檔案)
├ lib(其他 JAR 檔案)

WAR檔案(WebAppName.war)

單純把 WebApp 整個目錄壓縮成 JAR(ZIP) 檔案
檔名要與 WebAppName 相同，副檔名要是 .war

2.2

<!ELEMENT web-app (icon?, display-name?, description?, distributable?, context-param*, filter*, filter-mapping*, listener*, servlet*, servlet-mapping*, session-config?, mime-mapping*, welcome-file-list?, error-page*, taglib*, resource-env-ref*, resource-ref*, security-constraint*, login-config?, security-role*, env-entry*, ejb-ref*, ejb-local-ref*)>

deployment descriptor 內 servlet 的設定

Servlet 實體：<servlet> (設定在 <web-app> )
Servlet 名稱：<servlet-name> (設定在 <servlet>、<servlet-mapping> )
Servlet 類別：<servlet-class> (設定在 <servlet> )

<!ELEMENT servlet (icon?, servlet-name, display-name?, description?, (servlet-class|jsp-file), init-param*, load-on-startup?, run-as?, security-role-ref*)>

設定個別 servlet object 初始參數(設定在 <servlet> ) (1.3)

<!ELEMENT init-param (param-name, param-value, description?)>
註：可設定同一 <servlet-class>，用不同的 <servlet-name> 來提供不同的初始值

設定 WebApp (Servlet Context) 初始參數(設定在 <web-app>) (3.1)

<!ELEMENT context-param (param-name, param-value, description?)>

設定 URL 與 servlet 的對應關係(mapping)(設定在 <web-app> )

<!ELEMENT servlet-mapping (servlet-name, url-pattern)>

<servlet-name> 要與 <servlet> 中對應的 <servlet-name> 相同
<servlet-mapping> 要設定在 <servlet> 之後

Objectives 3.1~3.3 (約 5 題)

3.1

取得 Servlet Context 初始參數(deployment descriptor 內的 <context-param> ) (2.2)>

ServletContext interface:
String getInitParameter(String paramName)
Enumeration getInitParameterNames()

Servlet context listener

ServletContextListener interface:
void contextInitialized(ServletContextEvent e)
void contextDestroyed(ServletContextEvent e)

ServletContextEvent class:
ServletContext getServletContext()

Servlet context attribute linstener

ServletContextAttributeListener interface:
void attributeAdded(ServletContextAttributeEvent e)
void attributeRemoved(ServletContextAttributeEvent e)
void attributeReplaced(ServletContextAttributeEvent e)

ServletContextAttributeEvent class(extends ServletContextEvent):
ServletContext getServletContext()
String getName()
Object getValue()
Return the name and value of the attribute that was added, removed, or replaced.

Session attribute listener

HttpSessionAttributeListener interface:
void attributeAdded(HttpSessionBindingEvent e)
void attributeRemoved(HttpSessionBindingEvent e)
void attributeReplaced(HttpSessionBindingEvent e)

HttpSessionEvent class:
HttpSession getSession()

HttpSessionBindingEvent class(extends HttpSessionEvent):
HttpSession getSession()
String getName()
Object getValue()

參考：Linstener 請參考 3.3 的表格
註：javax.servlet.GenericServlet implements the javax.servlet.ServletConfig interface.

3.2

設定 WebApp (Servlet Context) 初始參數(設定在 <web-app> ) (2.2)

<!ELEMENT context-param (param-name, param-value, description?)>

Listener 的設定(設定在 <web-app> )

<!ELEMENT listener (listener-class)>
<listener> 要設定在 <servlet> 之前，<context-param> 之後
註：可寫不同的 listener class 實作不同的 Listener interface，也可只寫一個 listener class 實作所有需要的 Listener interface
參考：Linstener 請參考 3.3 的表格

3.3

分散式環境

There is one ServletContext for each web application on each JVM. 每個 WebApp 在每個 JVM 上只有一個 ServletContext
HttpSession exists on only one JVM at a time but may be migrated across the JVMs.
HttpSession attributes should implement the java.io.Serializable interface; otherwise, setAttribute() may throw an IllegalArgumentException.
ServletContext init parameters are available on all the JVMs.
ServletContext attributes are not visible across the JVMs.
ServletContextEvent, ServletContextAttributeEvent, and HttpSessionBindingEvent(implements HttpSessionAttributeListener) may not be propagated across the JVMs. Therefore, a distributable web application should not depend on the notification of changes to the attribute list of either ServletContext or HttpSession.

各種 Lintener 的比較

javax.servlet.ServletContextListener	void contextInitialized(ServletContextEvent sce) void contextDestroyed(ServletContextEvent sce)
javax.servlet.ServletContextAttrubuteListener	void attributeAdded(ServletContextAttributeEvent scae) void attributeRemoved(ServletContextAttributeEvent scae) void attributeReplaced(ServletContextAttributeEvent scae)
javax.servlet.http.HttpSessionListener	void sessionCreated(HttpSessionEvent se) void sessionDestroyed(HttpSessionEvent se)
javax.servlet.http.HttpSessionAttributeListener	void attributeAdded(HttpSessionBindingEvent se) void attributeRemoved(HttpSessionBindingEvent se) void attributeReplaced(HttpSessionBindingEvent se)

均要在 deployment descriptor 設定
分散式環境下，皆不適用

javax.sevlet.http.HttpSessionActivationListener	void sessionDidActivate(HttpSessionEvent se) void sessionWillPassivate(HttpSessionEvent se)
javax.servlet.http.HttpSessionBindingListener	void valueBound(HttpSessionBindingEvent event) void valueUnbound(HttpSessionBindingEvent event)

不用在 deployment descriptor 設定
使用事件驅動的 attribute 需要實作 Listener 介面
可在分散式環境下使用

補充

SRV.7.7.2 Distributed Environments
Within an application marked as distributable, all requests that are part of a session must handled by one virtual machine at a time. The container must be able to handle all objects placed into instances of the HttpSession class using the setAttribute or putValue methods appropriately. The following restrictions are imposed to meet these conditions:

The container must accept objects that implement the Serializable interface
The container may choose to support storage of other designated objects in the HttpSession, such as references to Enterprise JavaBean components and transactions.
Migration of sessions will be handled by container-specific facilities.

Objectives 4.1~4.3 (約 3 題)

4.1

使用 setStatus method 設定 status code

HttpServletResponse interface:
void setStatus(int statusCode)

A call to this method is ignored if the response is already committed.
Should only be used to set status codes that do not represent an error.
DOES NOT tigger the container to generate a default HTML page if no <error-code>/<error-page> mapping is specified. (4.2)

補充 by fishinwater：
This method is used to set the return status code when there is no error (for example, for the status codes SC_OK or SC_MOVED_TEMPORARILY). If there is an error, and the caller wishes to invoke an defined in the web applicaion, the sendError method should be used instead.

使用 sendError method 回傳 HTTP 錯誤

HttpServletResponse interface:
void sendError(int statusCode)
void sendError(int statusCode, String message)

Throws IllegalStateException if the response is already committed.
A response should be assumed to be committed after a call to this method.
Triggers the container to generate a default HTML page if an <error-code>/<error-page> mapping is not specified. (4.2)

4.2

自訂錯誤網頁：

方法一：

在 deployment descriptor 設定處理錯誤的錯誤網頁(設定在 <web-app> )

<!ELEMENT error-page ((error-code|exception-type), location)>

exception-type 要是 unique 的 package.Class；location must start with "/"
Only subclasses of ServletException, IOException, or RuntimeException can be thrown from the servlet code. All other business exceptions must be wrapped inside a ServletException.
javax.servlet.ServletOutputStream extends java.io.OutputStream.

補充 by ziyu：
雖然 Java 語法的限制，使得你只能丟出 IOExceptoin, ServletException, RuntimeException，但在 JSP 中，可以任意丟出 Exception，會自動以 ServletException 將之包住。故 <exception-type> 並無限制為此三類。
<error-page> 的判斷有2步，第一步是以instanceof來判斷，第二回是以 ServletException.getRootCause() 取出被自動包起來的 Exception object，因此若 <exception-type> 若是 ServletException，會使得所有非以上三類的 Exception 全被其截住，而產生意料外的 <error-page> 對映。

方法二：

取得 RequestDispatcher 物件 (1.6)

ServletRequest interface:
RequestDispatcher getRequestDispatcher(String path) ：path 可以是相對路徑或絕對路徑(以"/"開頭的路徑)
ServletContext interface:
RequestDispatcher getRequestDispatcher(String path) ：path 只能是絕對路徑(以 "/"開頭的路徑)
RequestDispatcher getNamedDispatcher(String name) ：name 只能是Servlet 的資源名稱

使用 RequestDispatcher 把 request 轉送到錯誤網頁(1.6)

void forward(ServletRequest req, ServletResponse res)

註：
Use RequestDispatcher.forward() to forward a request to an error-handler page. In case of an exception, set javax.servlet.error.exception_type, javax.servlet.error.request_uri, and javax.servlet.error.servlet_name in the request, and in case of an error condition, set javax.servlet.error.error_code, javax.servlet.error.request_uri, and javax.servlet.error.servlet_name in the request before forwarding the request.

Rules for exceptions thrown by an included or forwarded resource using the RequestDispatcher interface:

If the exception thrown by the included or forwarded resource is a RuntimeException, a ServletException, or an IOException, it is thrown as is by the include() or forward() method.
Otherwise, the exception is wrapped in a ServletException as the rootCause, and the ServletException is thrown by the include() or forward() method.

4.3

寫 log 的方法

ServletContext interface 與 GenericServlet class:
void log(String message)
void log(String message, Throwable t)

GenericServlet會把 servlet name 一起 log 進去，而 ServletContext 不會

註：

javax.servlet.GenericServlet implements java.io.Serializable, javax.servlet.Servlet, javax.servlet.ServletConfig
取得 ServletContext：
ServletContext ServletConfig.getServletContext()
ServletContext Servlet.getServletConfig().getServletContext()

Objectives 5.1~5.3 (約 4 題)

5.1

取得 session 物件(multiple requests、同一或不同 servlet、同一 WebApp)

HttpServletRequest interface:
HttpSession getSession() ： the same with getSession(true)
HttpSession getSession(boolean createFlag)

在 session 物件取得、設定、清除 object attribute

HttpSession interface:
Object getAttribute(String name)
void setAttribute(String name, Object value)
Enumeration getAttributeNames()
void removeAttribute(String name)

參考：attribute 請參考 1.4

處理 session 建立、摧毀的事件 (要在 deployment descriptor 設定，不適用分散式環境)

HttpSessionListener interface:
void sessionCreated(HttpSessionEvent e)
void sessionDestroyed(HttpSessionEvent e)

處理 session 內 attribute 增加、清除的事件 (不用在 deployment descriptor 設定，attribute 需實作此介面，適用分散式環境)

HttpSessionBindingListener interface:
void valueBound(HttpSessionBindingEvent e)
void valueUnbound(HttpSessionBindingEvent e)

參考：Linstener 請參考 3.3 的表格

使 session 無效

HttpSession interface:
void invalidate()

5.2

設定 session 的自動 timeout 時間
1. HttpSession.setMaxInactiveInterval(int seconds)：單位秒，當值 < 0 時表示不會自動 timeout，只對呼叫此 method 的 session 作用
2.在 deployment descriptor 設定 (設定在 <web-app> 的 <session-config> 裡)
<!ELEMENT session-config (session-timeout?)>：單位分鐘，當值 <= 0 時表示不會自動 timeout，對所有 session 作用
‧ HttpSession.setMaxInactiveInterval(int seconds) 比在 deployment descriptor 設定 <session-timeout> 來得優先
‧ 要使 session 無效，除非 session 自動 timeout 或呼叫 session.invalidate()

5.3

要使用 URL-rewriting
HttpServletResponse interface:
String encodeURL(String originalUrl)
‧ 無法使用 session 時，在 URL 後加上 session ID；可使用 session 時，則不變更 URL

要使用 URL-rewriting 來重新導向別的網頁：HttpServletResponse.sendRedriect(String url) (1.3)
HttpServletResponse interface:
String encodeRedirectURL(String originalUrl)

註：要把靜態 HTML 改成 servlet 或 JSP，裡面的 URL 才能使用 URL-rewriting

補充 by fishinwater：
All URLs sent to the HttpServletResponse.sendRedirect method should be run through this method. Otherwise, URL rewriting cannot be used with browsers which do not support cookies.

Objectives 6.1~6.3 (約 4 題)

6.1

Authentication
Authentication is the mechanism by which parties identify themselves to each other and prove that they have the rights to access a system. Authentication indicates who you are (or who you represent).
詢問 username 與 password 來驗證使用者身份

Authorization
Authorization ensures that data and resources are only made available to parties that are authorized to use them.
控制使用者的存取權

Authentication 與 Authorization的比較
Authentication只是讓你登入用的,存取權是由 Authorization 來控制

Data integrity
Data integrity means that the data has not been modified on its route from the client to the server(or vice versa).
確認傳輸的資料沒有被變更(常使用加密)

Confidentiality
Making sure that unintended parties cannot make use of the data. (常使用加密)

Auditing
Auditing means keeping a log of who does what within an application.
使用 log 來記錄動作

Malicious code
Code aimed at breaking the security rules of a web application.
惡意程式，可使用數位簽章(signed code)來避免

Web site attacks
A web site attack is the name given to any attack against a web site. Examples of this could include Denial of Service attacks or just plain password hacking.
大量 request(denial-of-service) 佔據系統資源，可使用 monitoring access patterns 來避免

6.2

在 deployment descriptor 設定安全限制(設定在 <web-app> )
<!ELEMENT security-constraint (display-name?, web-resource-collection+, auth-constraint?, user-data-constraint?)>

需要安全限制的 Web resource(設定在 <security-constraint> )
<!ELEMENT web-resource-collection (web-resource-name, description?, url-pattern*, http-method*)>
<http-method> 的值：GET、POST 等，限制只能使用該 HTTP METHOD 來存取被保護的 web resource，沒設表示所有 HTTP METHOD 都被保護

使用者限制(設定在 <security-constraint> )
<!ELEMENT auth-constraint (description?, role-name*)>
<role-name> 的值：* 或是有設定在 <security-role> 裡的 <role-name>

資料傳輸限制(設定在 <security-constraint> )
<!ELEMENT user-data-constraint (description?, transport-guarantee)>
<transport-guarantee> 的值： NONE implies HTTP；CONFIDENTIAL, INTEGRAL imply HTTPS

Authentication mechanism：The login configuration(設定在 <web-app> )
<!ELEMENT login-config (auth-method?, realm-name?, form-login-config?)>
<auth-method> 的值：BASIC、FORM、DIGEST、CLIENT-CERT

設定 form based login 的 login and error pages(設定在 <login-config> )
<!ELEMENT form-login-config (form-login-page, form-error-page)>
註：If form based authentication is not used, these elements are ignored.

A security role(設定在 <web-app> )
<!ELEMENT security-role (description?, role-name)>

註：Programmatic security requires role names that are hard-coded in the servlet to be specified in the security-role-ref element(設定在 <servlet> )
<!ELEMENT security-role-ref (description?, role-name, role-link?)>
Manager will be hard-coded in the servlet while supervisor is the actual role name in the deployment environment.
請參考 Manning SCWCD Exam Study Kit
API：HttpServletRequest interface:
String getRemoteUser()
java.security.Principal getUserPrincipal()
boolean isUserInRole(String role)

6.3

BASIC
With BASIC authentication, passwords and usernames are sent to the server in plaintext (this is unencrypted).
Username and password information is gathered by popping up a box on the client.
Performed by sending the username and password in Base64 encoding.

DIGEST
With DIGEST authentication, passwords are encrypted using a simple hashing algorithm.
The username is still sent as plaintext. Username and password information is gathered by popping up a box on the client.
Performed by sending a digest of the password in an encrypted form.

FORM
FORM-based authentication allows the user to enter their username and password using a custom-built form.
This allows the login functionality to be customized for the application.
Performed by sending username and password in Base64 encoding. The username and password are captured using a customized HTML FORM.

CLIENT-CERT
End-user authentication using HTTPS (HTTP over SSL) is a strong authentication mechanism. This mechanism requires the user to possess a Public Key Certificate (PKC). Currently, PKCs are useful in e-commerce applications and also for a single sign-in from within the browser. Servlet containers that are not J2EE-compliant are not required to support the HTTPS protocol.

Authentication type	Advantages	Disadvantages
BASIC	‧ Very easy to set up ‧ Supported by all browsers	‧ It is not secure, since the username and password are not encrypted. ‧ You cannot customize the look and feel of the dialog box.
DIGEST	‧ Secure	‧ Not supported by all browsers
FORM	‧ Easy to set up ‧ Supported by all browsers ‧ Custiomiezd look and feel	‧ It is not secure, since the username and password are not encrypted unless HTTPS is used.
CLIENT-CERT	‧ Very secure ‧ Supported by all browsers	‧ Costly to implement

‧ The Http specification only defines BASIC and DIGEST authentication mechanisms.

Objectives 7.1~7.3 (約 3 題)

7.1

Variable/Attribute scope	Thread Safe in Multi-Threaded Model	Thread Safe in Single-Threaded Model
Local Variables	Yes	Yes
Instance Variables	No	Yes
Class Variables	No	No
Request Attributes	Yes	Yes
Session Attributes	No	No
Context Attributes	No	No

註：
Instance Variables 只在 Single-Threaded Model thread-safe，原因在於 Multi-Threaded Model 讓每個 request 在同一 servlet 實體叫起一個 service() thread 同時跑，而 Single-Threaded Model 是叫起不同的 servlet 實體來跑

Class Variables 一定不是 thread-safe，因為在 Single-Threaded Model，雖然是叫起不同的 servlet 實體，但是更改的還是同一個值，所以會有 concurrency 問題

Request Attribute 一定是 Thread Safe，因為他的 scope 就是在那 method 裡，不可能在中間被別的 method 裡被更動

Session 一定不是 Thread Safe，原因在於使用者可能另開視窗，造成多個視窗同一 session 的問題(同browser)

Context Attributes 一定不是 Thread Safe，因為它就是那個 Web App 的一個共同記憶體，都可能會被更改

7.2

Multi-Threaded Model
‧ Only one instance of the servlet class is created.
‧ Multiple threads can execute the service() method sumultaneously.
‧ Requests are serviced in parallel by a single instance.

Single-Threaded Model
‧ Multiple instances of the servlet class may be created.
‧ Only one thread executes the service() method of one instance.
‧ Mulitple requests may be serviced concurrently by multiple instances.
‧ One instance services only one request at a time.

7.3

Implements javax.servlet.SingleThreadModel interface ensures that only one thread executes the service() method at a time.
盡量不要實作 javax.servlet.SingleThreadModel 介面
實作 javax.servlet.SingleThreadModel 只保證 Instance Variables 能夠 thread safe

Objectives 8.1~8.7 (約 8 題)

8.1、8.3

	JSP tag	XML-based tag
Directive	<%@ directive {attribute="value"}* %> Example: <%@ page attribute list %> <%@ include file="relativeURL" %> <%@ taglib prefix="prefix" uri="uri" %>	<jsp:directive.page isErrorPage="true" /> <jsp:directive.include file="relativeURL" />
Declaration	<%! declaration; %> Example: <%! int count; %> <%! int getCount(){ return count; } %>	<jsp:declaration> int j = 0; </jsp:declaration>
Scriptlet	<% seriptlet; %>	<jsp:scriptlet> if(x > 10){ </jsp:scriptlet>
Expression	<%= expression %> Example: <%= request.getParameter("paramName") %>	<jsp:expression> new java.util.Date() </jsp:expression>
Action	無	<jsp:action {attribute="value"}* /> <jsp:action {attribute="value"}* ></jsp:action>

‧ All XML-based pages should have a root tag ramed <jsp:root>. Thus, the page ends with </jsp:root>.
‧ Unknown attributes or invalid values result in errors and are caught during the translation phase.
‧ The standard JSP tags and XML-based tags cannot be mixed within a single JSP page.
‧ A page written in one syntax format can, however, include or forward to a page in the other syntax format by using either directive or action.
‧ <jsp:directive.include file="./header.jsp" /> 與 <jsp:include page="page.jsp" /> 是不同的
一個是 include directive 另一個是 include action (9.1)

Directive、Action、XML syntax format：
‧ Tag names, their attributes, and their values are case sensitive.
‧ The value must be enclosed within a pair of single or double quotes.
‧ A pair of single quotes is equivalent to a pair of double quotes.
‧ There must be no space between the equals sign (=) and the value.

補充 by lazymeg：
Request-Time Expression 的兩種不同形式

JSP tag	XML-based tag
<jsp:param name="name" value="<%= RTExpr %>" />	<jsp:param name="name" value="%= RTExpr %" />

8.2

Directive
Specifies translation time instructions to the JSP engine.
Provide global info across the page independent of any request.
Three types of directives: page, taglib and include
‧ page directives can be placed anywhere in a page but apply to the entire translation unit.
註：taglib 沒有對應的 XML-based tag, Taglibs are specified in <jsp:root> (參考11.2)

Declaration
在 _jspService()外宣告 class/instance variable (7.1)、 class/instance method

Scriptlet
Used for writing free-form Java code.
‧ 若有宣告 variable，該變數會是在 _jspService()裡的 local variable
‧ 不能宣告 method

Expression
Used as a shortcut to print values in the generated page.
‧ Expressions must not end with a semicolon.

Action
Provides request time instructions to the JSP engine.
Action 有：<jsp:fallback>、<jsp:forward>(9.1)、<jsp:getProperty>(10.1)、<jsp:include>(9.1)、<jsp:param>、<jsp:params>、<jsp:plugin>、<jsp:setProperty>(10.1)、<jsp:useBean>(10.1)

補充：
<jsp:fallback> 只能用在 <jsp:plugin> 內，沒有 attribute

<jsp:param name="name" value="value" /> 可用在 <jsp:forward>、<jsp:include>、<jsp:params> 內，value 可以是 request-time value

<jsp:params> 只能用在 <jsp:plugin> 內，沒有 attribute

<jsp:plugin type="applet" code="Applet.class" codebase="applet" jreversion="1.2" width="160" heifht="150" >
<jsp:params>
<jsp:param name="bgcolor" value="ccddff" />
</jsp:params>
<jsp:fallback>
Plugin tag OBJECT or EMBED not supported by browser.
</jsp:fallback>
</jsp:plugin>
● mandatory attribute：code、codebase、type
● optional attribute：align、archive、height、hspace、iepluginurl、jreversion、name、nspluginurl、title、vspace、width

Comment
Used to comment out parts of JSP code.
補充：可見到註解的地方：

	<%-- JSP註解 --%>	<% //JAVA註解 %>	<!-- HTML註解 -->
JSP檔案	Yes	Yes	Yes
轉譯後的 JAVA檔	No	Yes	Yes
回傳給 client 的 HTML檔	No	No	Yes

8.4

page directive attribute：import、session、errorPage、isErrorPage

Import a Java class into the JSP page
<%@ page import="java.util.Date" %>
‧ "java.lang.*,javax.servlet.*,javax.servlet.http.*,javax.servlet.jsp.*" 會被自動 import

Declare that a JSP page exists within a session
<%@ page session="true" %>：預設為 true

Declare that a JSP page uses an error page
<%@ page errorPage="./errorPage.jsp" %>：預設為 null

Declare that a JSP page is an error page
<%@ page isErrorPage="true" %>：預設為 false

註：page directive 中，只有 import 可以宣告多次

8.5

JSP page life-cycle (1.5 Servlet life-cycle)
1. Page translation
2. Page compilation
3. Load class
4. Create instance
5. Call void jspInit()： call only once
6. Call void _jspService(HttpServletRequest req, HttpServletResponce res) throws ServletException, IOException：
called multiple times, once for every request for this page
7. Call void jspDestroy()：call only once

‧ jspInit() and jspDestroy() are defined in the javax.servlet.jsp.JspPage interface.
‧ _jspService() is defined in the javax.servlet.jsp.HttpJspPage interface.
‧ JSP declarations are used to declare(override) jspInit() and jspDestroy().
‧ We never declare(override) _jspService() explicitly. The JSP engine automatically declares it.

8.6

Implicit objects	Type	Purpose, Function, or Uses
request	javax.servlet.http.HttpServletRequest	Passed in as a parameter to _jspService(). Used for getting HTTP header information, cookies, parameters, etc. Also used for getting and setting attributes into the request scope.
response	javax.servlet.http.HttpServletResponse	Passed in as a parameter to _jspService(). Used to send a response to the client. Used for setting HTTP header information, cookies, etc.
out	javax.servlet.jsp.JspWriter	Used for writing data to the output stream.
session	javax.servlet.http.HttpSession	Used for storing and retrieving session related information and sharing objects across multiple requests and pages within the same HTTP session.
config	javax.servlet.ServletConfig	Used to retrieve initialization parameters for a JSP page.
application	javax.servlet.ServletContext	Used for storing and retrieving application related infotmation and sharing objects across multiple sessions, requests, and pages within the same web application.
page	java.lang.Object	Refers to the generated Servlet class. Not used much because it is declared as type java.lang.Object. (使用前要 cast)
pageContext	javax.servlet.jsp.PageContext	Used for storing and retrieving page-related information and sharing objects within the same translation unit and same request. Also used as a convenience class that maintains a table of all the other implicit objects.
exception	java.lang.Throwable	Only available in pages that have the page directive isErrorPage set to true.

參考：用 PageContext 取得 implict variable 請參考 12.7

8.7

conditional statement
<% if(someCondition){ %>
regular HTML/JSP code
<% }else{ %>
regular HTML/JSP code
<% } %>

iteration statement
<% for(int i = 0; i < 10; i++){ %>
regular HTML/JSP code
<% } %>

Objectives 9.1 (約 1 題)

9.1

include directive：靜態 include
<%@ include file="relativeURL" %>
<jsp:directive.include file="relativeURL" />
● relativeURL 不能指向 servlet
● 不可加參數 (query string)，只可使用原 request 內的參數

include action：動態 include
<jsp:include page="relativeURL" flush="true|false" />
● relativeURL 要是 page(HTML, JSP 等)，不可是 url 或 file，可以指向 servlet，接受 request-time value
● flush is optional，預設 false
● <jsp:include> actions are translated and evaluated on their own
● 可傳參數，且參數可以是動態產生的，也可使用原 request 內的參數

Example: (8.2)
<jsp:include page="somePage.jsp">
<jsp:param name="name1" value="<%= someExpr1 %>"/>
<jsp:param name="name2" value="<%= someExpr2 %>"/>
</jsp:include>

● Three equivalent constructs：

Construct 1

<%
RequestDispatcher rd = request.getRequestDispatcher("other.jsp");
rd.include(request, response);
%>

Construct 2

<%
pageContext.include("other.jsp");
%>

Construct 3

<jsp:include page="other.jsp" flush="true" />

forward action：動態 forward
<jsp:forward page="relativeURL" />
● relativeURL 要是 page(HTML, JSP 等)，不可是 url 或 file，可以指向 servlet，接受 request-time value
● 可傳參數，且參數可以是動態產生的，也可使用原 request 內的參數
● Three equivalent constructs：

Construct 1

<%
RequestDispatcher rd = request.getRequestDispatcher("other.jsp");
rd.forward(request, response);
%>

Construct 2

<%
pageContext.forward("other.jsp");
%>

Construct 3

<jsp:forward page="other.jsp" />

註：
include directive、include action、forward action：都是使用相對路徑
include directive 不能指向 servlet；include action、forward action可以指向 servlet

API：PageContext class:
abstract void forward(String relativeUrlPath) throws ServletException, java.io.IOException
abstract void include(String relativeUrlPath) throws ServletException, java.io.IOException

Objectives 10.1~10.3 (約 5 題)

10.1

<jsp:useBean id="name" scope="page|request|session|application" typeSpec />
typeSpec::= class="className" |
            class="className" type="typeName" |
            beanName="beanName" type="typeName" |
            type="typeName"
● mandatory attribute：id； optional attribute：scope(預設 page)；至少要提供 class 或 type；beanName 可以是 request-time value
● The body of the <jsp:useBean> action can be used to initialize its properties.

Example:
<jsp:useBean id="address" scope="session" class="AddressBean">
<jsp:setProperty name="address" property="street" value="123 Main" />
</jsp:useBean>

● The beanName attribute can also be a JSP expression.
● beanName can be used to instantiate a class or a serialized object whereas class is only for a class.
● class uses the new keyword to instantiate a class and beanName uses java.beans.Beans.instantiate()
● Using beanName and class together is illegal.

<jsp:setProperty name="beanName" prop-expr />
prop-expr::= property="*" |
             property="propertyName" |
             property="propertyName" param="parameterName" |
             property="propertyName" value="propertyValue" |
● mandatory attribute：name、property； Using param and value together is illegal；propertyValue 可以是 request-time value
● The name attribute must refer to a bean that is already declared using a useBean action.
● Examples:

Sets all the properties for which there is a matching parameter in the request
<jsp:setProperty name="aName" property="*" />

Sets aProp using the parameter of the same name in the request
<jsp:setProperty name="aName" property="aProp" />

Sets aProp using the parameter named aParam in the request
<jsp:setProperty name="aName" property="aProp" param="aParam" />

Sets aProp to the value aValue
<jsp:setProperty name="aName" property="aProp" value="aValue" />

Sets aProp to the value returned by the expression
<jsp:setProperty name="aName" property="aProp" value="<%= JSPExpression %>" />

<jsp:getProperty name="name" property="propertyName" />
● mandatory attribute：name、property
● <jsp:getProperty> prints out the value.

10.2

JSP scopes：page(PageContext)、request(HttpServletRequest)、session(HttpSession)、application(ServletContext) (1.4 Servlet scopes)

10.3

可以存取 JavaBean 的方式

JSP scriptlet
JSP expression
JSP standard action (<jsp:uesBean> 等)
custom action 或 custom tag

註：Once the use of a JavaBean is declared using a <jsp:useBean> action, a variable by the given name is automatically declared in the servlet code.Therefore, besides the setProperty and getProperty actions, the bean can alse be accessed through this variable in the scription elements.

Example:
<jsp:useBean id="user" class="UserBean" scope="session" />
<%
//The bean is used in a scriptlet here. You can call methods on the object referred to by the user variable.
user.initialize();
out.println(user.getName());
%>

Objectives 11.1~11.3 (約 3 題)

11.1

在 deployment descriptor 設定 tag library(設定在 <web-app> )
<!ELEMENT taglib (taglib-uri, taglib-location)>

Example:
<web-app>
<taglib>
    <taglib-uri>/myTag</taglib-uri>
    <taglib-location>
      /WEB-INF/tld/mytag.tld
    </taglib-location>
</taglib>
</web-app>

● Each taglib element maps one URI to one location.
● The value of <taglib-uri> can be an absolute URI, a root-relative URI or a non-root-relative URI.
● The value of <taglib-location> can be a root-relative URI or a non-root-relative URI. It cannot be an absolute URI.
● The value of <taglib-uri> must be unique in the deployment descriptor.
● The value of <taglib-location> must point to a valid TLD resource path. It can be either a TLD file or a JAR file containing the TLD file at location META-INF/taglib.tld.

11.2

taglib directive

<%@ taglib uri="/taglibURI" prefix="prefix" %>
註：prefix不可使用 jsp、jspx、java、servlet、sun、sunw
uri 的值要與 <!ELEMENT taglib (taglib-uri, taglib-location)> 中的 <taglib-uri> 相同

補充 by Michael Deng：
When the JSP engine parses a JSP file and encounters a taglib directive, it checks its taglib map to see if a mapping exists for the uri attribute of the taglib directive:

If the value of the uri attribute matches any of the <taglib-uri> entries, the engine uses the value of the corresponding <taglib-location> to locate the actual TLD file.
If the <taglib-location> value is a root-relative URI (that is, it starts with a /), the JSP engine assumes the location to be relative to the web application's document root directory.
If the <taglib-location> value is a non-root relative URI (that is, it starts without a /), the JSP engine prepends /WEB-INF/ to the URI and assumes the location to be relative to the web application's document root directory.
If the value of the uri attribute of the taglib directive does not match any of the <taglib-uri> entries, then the following three possibilities arise:

If the specified uri attribute is an absolute URI, then it is an error and is reported at translation time.
If the specified uri attribute is a root-relative URI, it is assumed to be relative to the web application's document root directory.
If the specified uri attribute is a non-root-relative URI, it is assumed to be relative to the current JSP page.

● In XML syntax, a tag library is introduced in the <jsp:root> element：
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page"
xmlns:prefix="uri" version="1.2" >
...JSP PAGE...
</jsp:root>

補充 by fishinwater：xmlns 和 version 都是mandatory的，xmlns 和 <%@ taglib %> 一樣可以多個

11.3

An empty custom tag

<prefix:tagName/> 或 <prefix:tagName></prefix:tagName>

A custom tag with attributes

<prefix:tagName attribute="value"/> 或 <prefix:tagName attribute="value"></prefix:tagName>

A custom tag that surrounds other JSP code

<prefix:tagName attribute="value">
<% //JSP code %>
</prefix:tagName>

Nested custom tags

<prefix:tagName1 attribute="value">
<prefix:tagName2/>
</prefix:tagName1>
● You cannot nest a custom tag in the attribute list of another custom tag like this：
<test:tag1 name="<test:tag2 />" />
In this case, the JSP engine may assume "<test:tag2 />" as a String value passed to the name attribute. It will not execute tag2.

Objectives 12.1~12.8 (約 8 題)

Tag Library Descriptor element names

12.1

<!ELEMENT taglib (tlib-version, jsp-version, short-name, uri?, display-name?, small-icon?, large-icon?, description?, validator?, listener*, tag+) >

<tag> 設定在 tag library descriptor (TLD,taglib.tld) 的 <taglib> 內
<!ELEMENT tag (name, tag-class, tei-class?, body-content?, display-name?, small-icon?, large-icon?, description?, variable*, attribute*, example?) >
The name of the tag：<name>
The class of the tag use：<tag-class>
The type of content that the tag accepts：<body-content> (值可以是 empty、JSP、tagdependent)
Any attributes of the tag：<attribute>

12.2

<attribute> 設定在 tag library descriptor (TLD,taglib.tld) 的 <tag> 內
<!ELEMENT attribute (name, required? , rtexprvalue?, type?, description?) >
The name of a tag attribute：<name>
Whether a tag attribute is required：<required>，預設 false
Whether or not the attribute's value can be dynamically specified：<rtexprvalue>，預設 false
● For each attribute, the container calls the setXXX() method.

12.3

<body-content> 設定在 tag library descriptor (TLD,taglib.tld) 的 <tag> 內
Empty tag：<body-content>empty</body-content>
Custom tag that surrounds other JSP code：<body-content>JSP</body-content> (預設)
Custom tag that surrounds content that is used only by the tag handler：<body-content>tagdependent</body-content>
● If <body-content> is specified as empty, the actual tag usage cannot have a body.

12.4

int doStartTag() throws JspException
Tag.doStartTag() is called after the JSP engine completely parses the opening tag. Before calling doStartTag(), the JSP engines calls the following methods:
1. setPageContext()
2. setParent()
3. setter methods for attributes

int doAfterBody() throws JspException
IterationTag.doAfterBody() is called after the JSP engine completely evaluates the entire body of the tag.
This happens the first time only if：
1. The tag implements IterationTag and doStartTag() returns EVAL_BODY_INCLUDE.
2. The tag implements BodyTag and doStartTag() returns EVAL_BODY_INCLUDE.
3. The tag implements BodyTag and doStartTag() returns EVAL_BODY_BUFFERED.
● doAfterBody() is not called for tags that implement only the Tag interface.

The body of the tag is evaluated again(repeatedly), that is, the doAfterBody() is called repeatedly, only if:
1. The tag implements IterationTag and the previous call to doAfterBody() returns EVAL_BODY_AGAIN.
2. The tag implements BodyTag and the previous call to doAfterBody() returns EVAL_BODY_AGAIN.
3. The tag implements BodyTag and the previous call to doAfterBody() returns EVAL_BODY_BUFFERED.

int doEndTag() throws JspException
Tag.doEndTag() is always called at the end of processing a tag.

12.5

Method	Return Value/Type
Tag.doStartTag()	SKIP_BODY、EVAL_BODY_INCLUDE， (EVAL_BODY_BUFFERED， implements BodyTag)
IterationTag.doAfterBody()	SKIP_BODY、EVAL_BODY_AGAIN， (EVAL_BODY_BUFFERED， implements BodyTag)
Tag.doEndTag()	SKIP_PAGE、EVAL_PAGE
PageContext.getOut()	javax.servlet.jsp.JspWriter

● doEndTag() is always called at the end of processing a tag regardless of the interfaces implemented and regardless if the return values from doStartTag() and doAfterBody().
● At request time, if the doStartTag() returns EVAL_BODY_BUFFERED，PageContext.getOut() returns an object of type BodyContent (which extends JspWriter) when it is called from within the body of the tag.

12.6

SKIP_BODY：This skips body evaluation and is a valid return value for doStartTag() and doAfterBody().
EVAL_BODY_INCLUDE：This evaluates the body of the tag and is a valid return value for doStartTag().
SKIP_PAGE：This skips the rest of the page and is a valid return value for doEndTag().
EVAL_PAGE：This continues evaluating the page and is a valid return value for doEndTag().
EVAL_BODY_AGAIN：This indicates that we require re-evaluation of the tag's body.This can be returned from doAfterBody().
EVAL_BODY_BUFFERED：This requests the creation of new buffer - a BodyContent object - in which we evaluate the body of this tag. This can be returned from doStartTag() when it implements the BodyTag interface. This is an illegal return value for doStartTag() when the class does not implement BodyTag.

Constants	Tag.doStartTag()	IterationTag.doAfterBody()	Tag.doEndTag()
EVAL_BODY_INCLUDE	Includes the body of the tag for the first time.
EVAL_BODY_AGAIN		Includes the body of the tag for the second and further times in a loop.
EVAL_BODY_BUFFERED	Allow processing of the tag body for the first time. Uses buffering. Valid only for tags implementing the BodyTag interface.	Allows processing of the tag body repratedly in a loop. Valid only for tags implementing the BodyTag interface.
SKIP_BODY	Does not include or process the body of the tag even for the first time.	Does not include or process the body of the tag the next time.
EVAL_PAGE			Continue processing the rest of the page.
SKIP_PAGE			Do not process the rest of the page. Skip everything from the end of this tag to the end of the page.

12.7

用 PageContext class 取得 implicit variable

Implicit variable	Using convenience methods	Using constants	Return Type
application	getServletContext()	getAttribute(PageContext.APPLICATION)	javax.servlet.ServletContext
session	getSession()	getAttribute(PageContext.SESSION)	javax.servlet.http.HttpSession
request	getRequest()	getAttribute(PageContext.REQUEST)	javax.servlet.ServletRequest
response	getResponse()	getAttribute(PageContext.RESPONSE)	javax.servlet.ServletResponse
out	getOut()	getAttribute(PageContext.OUT)	javax.servlet.jsp.JspWriter
config	getServletConfig()	getAttribute(PageContext.CONFIG)	javax.servlet.ServletConfig
page	getPage()	getAttribute(PageContext.PAGE)	java.lang.Object
pageContext		getAttribute(PageContext.PAGECONTEXT)	javax.servlet.jsp.PageContext
exception	getException()	getAttribute(PageContext.EXCEPTION)	java.lang.Exception

註：這些 method 都為 abstract

取得、設定、清除 JSP page's attributes

PageContext class:
Object getAttribute(String name)
Object getAttribute(String name, int scope)
void setAttribute(String name, Object attribute)
void setAttribute(String name, Object attribute, int scope)
void removeAttribute(String name)
void removeAttribute(String name, int scope)
Object findAttribute(String name) ： Searches for the named attribute in page, request, session (if valid), and application scope(s) in order and returns the value associated or null.
int getAttributesScope(String name)
Enumeration getAttributeNamesInScope(int scope)

scope：application(PageContext.APPLICATION_SCOPE)、session(PageContext.SESSION_SCOPE)、request(PageContext.REQUEST_SCOPE)、page(PageContext.PAGE_SCOPE)

參考：attribute 請參考 1.4

12.8

Get the direct parent

Tag interface:
Tag getParent()

Get any parent tag

TagSupport class(implements IterationTag, java.io.Serializable, Tag):
Tag getParent()
static Tag findAncestorWithClass(Tag from, java.lang.Class klass)

TagSupport.findAncestorWithClass(Tag from, java.lang.Class klass) works outward within the nested tags and gets the instance of a given class type that is closest to the given tag instance.

註：Tag、TagSupport 有 void setParent(Tag parentTag)

Objectives 13.1~13.2 (約 4 題)

13.1、13.2

Value Objects(VO)

Value Objects can be used best in scenarios where clients need to exchange data with Enterprise JavaBeans. Applications need to transfer data across tiers.

Reduces network traffic and improves user response time because of decreased latency.
Reduces load on Enterprise JavaBeans because the value objects are cached on the client by the Value Object instance.

Model-View-Controller(MVC)

MVC is typically used in scenarios where an application needs to support many diverse client types that use the same back-end system.
The pattern also reduces coupling between presentation logic and business logic.

Enhances reusability be decoupling application functionality from presentation.
Enhances flexibility and eases support for new types of clients.
Improves extensibility by simplifying the design.
Allows the same data to be accessed/viewed in multiple clients.
Eases maintenance and reduces duplication of code.
The design becomes clearer.

Data-Access Objects(DAO)

They are used in cases where the method of data access varies due to data being persisted in various different forms. For example, XML, relational databases, comma separated files, legacy systems, and so on.
Furthermore, accessing data using the same mechanism can vary due to vendor-specific implementations.

Enhanced extensibility.
Provides resource vendor and implementation independence.
Provides greater deployment flexibility.
Separates business logic from data access logic.

Business Delegate

The client communicates across a network and thus is expected to cope with the complexity of remote method invocations. An example of this complexity is the necessity to catch RemoteExceptions that might be thrown from EJB. This pattern allows the client to ignore the fact that calls are remote by handling the complexities of remote communication.

Reduces coupling.
Eases maintenance.
Hides the details of the underlying service.

Issues	Pattern
Small object Grouped information Read-only data Reduce network traffic Improve response time Transfer data across networked tiers	Value Object
Flexible design Provide services to different clients: web client, WAP client, etc. Multiple views, such as HTML or WML Single controller	Model-View-Controller(MVC)
Uniform access to the database Transparent access to the database Centralized access to the database Reduce dependency on the type of database Reduce dependency on the database access mechanism Reduce coupling between the enterprise beans and the database Reduce coupling between the business objects and the database Easier migration of data from one database to another Multiple data sources, such as database systems in the intranet, database systems	Data Access Object(DAO)
Reduce coupling between presentation and business tiers Proxy for the client Client-side facade Cache business service references for presentation-tier components Cache business service results for presentation-tier components Encapsulate business service lookup Encapsulate business service access Decouple clients from business service API	Business Delegate
Dispatch requests Manage workflow of a web application Manage the sequence of steps Manage use cases	Front Controller

Pattern/Important concepts	Potential benfits
A Value Object is a small-sized serializable Java object that is used for transferring data over the network in a distributed application.	Less communication overhead Fewer number of remote calls Reduction in network traffic Increased response time
The Model-View-Controller design pattern is applicable in situations where the same data(Model) is to be presented in different formats(Views), but is to be managed centrally by a single controlling entity(Controller).	Flexible design Centrally managed data Multiple ways of presentation
A Data Access Object provides other application components with a clean, simple, and common interface to access the data from various multiple data stores. DAO talks to the actual underlying database and takes care of the different types of data access mechanism.	Reduced dependency of other components on the datails of using the database Access to database unified, centralized, and made transparent to the business-tier components Reduced dependency on type of database Reduced dependency on database access mechanism Reduced coupling between the enterprise beans and the database Reduced coupling between the business objects and the database Migration of data from one database to another made easier
A Business Delegate is an object that communicates with the business service components on behalf of the client components. The client-side components delegate the work of accessing the business services to the Business Delegate object.	Reduced coupling between presentation and business tiers Cached business service results for presentation-tier components Business service lookup encapsulated Business service access encapsulated Decoupled clients from business serviceAPI

補充：J2EE Patterns Catalog

記：與 header (1.3)、redirect (1.3)、HTTP status code (4.1)、session (5.1) 有關的，均使用以 Http 開頭的 interface (HttpServletRequest、HttpServletResponse、HttpSession) 其他都使用不以 Http 開頭的 interface (ServletRequest、ServletResponse、ServletConfig)