Servlet (Java EE 5)

servlet 是运行在 Web 服务器中的小型 Java 程序。servlet 通常通过 HTTP（超文本传输协议）接收和响应来自 Web 客户端的请求。

要实现此接口，可以编写一个扩展 javax.servlet.GenericServlet 的一般 servlet，或者编写一个扩展 javax.servlet.http.HttpServlet 的 HTTP servlet。

此接口定义了初始化 servlet 的方法、为请求提供服务的方法和从服务器移除 servlet 的方法。这些方法称为生命周期方法，它们是按以下顺序调用的：

1. 构造 servlet，然后使用 init 方法将其初始化。
2. 处理来自客户端的对 service 方法的所有调用。
3. 从服务中取出 servlet，然后使用 destroy 方法销毁它，最后进行垃圾回收并终止它。

除了生命周期方法之外，此接口还提供了 getServletConfig 方法和 getServletInfo 方法，servlet 可使用前一种方法获得任何启动信息，而后一种方法允许 servlet 返回有关其自身的基本信息，比如作者、版本和版权。

Defines methods that all servlets must implement.

A servlet is a small Java program that runs within a Web server. Servlets receive and respond to requests from Web clients, usually across HTTP, the HyperText Transfer Protocol.

To implement this interface, you can write a generic servlet that extends javax.servlet.GenericServlet or an HTTP servlet that extends javax.servlet.http.HttpServlet.

This interface defines methods to initialize a servlet, to service requests, and to remove a servlet from the server. These are known as life-cycle methods and are called in the following sequence:

1. The servlet is constructed, then initialized with the init method.
2. Any calls from clients to the service method are handled.
3. The servlet is taken out of service, then destroyed with the destroy method, then garbage collected and finalized.

In addition to the life-cycle methods, this interface provides the getServletConfig method, which the servlet can use to get any startup information, and the getServletInfo method, which allows the servlet to return basic information about itself, such as author, version, and copyright.

Author:

Various

See Also:

GenericServlet, HttpServlet

public void init(ServletConfig config) throws ServletException
由 servlet 容器调用，指示将该 servlet 放入服务。

servlet 容器仅在实例化 servlet 之后调用 init 方法一次。在 servlet 可以接收任何请求之前，init 方法必须成功完成。

servlet 容器无法将 servlet 放入服务，如果 init 方法：

1. 抛出 ServletException
2. 未在 Web 服务器定义的时间段内返回

config	包含 servlet 的配置和初始化参数的 ServletConfig 对象
Throws	ServletException: 如果发生妨碍 servlet 正常操作的异常
See also	javax.servlet.UnavailableException, getServletConfig

英文文档：

init

void init(ServletConfig config)
          throws ServletException

Called by the servlet container to indicate to a servlet that the servlet is being placed into service.

The servlet container calls the init method exactly once after instantiating the servlet. The init method must complete successfully before the servlet can receive any requests.

The servlet container cannot place the servlet into service if the init method

1. Throws a ServletException
2. Does not return within a time period defined by the Web server

Parameters:

config - a ServletConfig object containing the servlet's configuration and initialization parameters

Throws:

ServletException - if an exception has occurred that interferes with the servlet's normal operation

See Also:

UnavailableException, getServletConfig()

public ServletConfig getServletConfig()
返回 ServletConfig 对象，该对象包含此 servlet 的初始化和启动参数。返回的 ServletConfig 对象是传递给 init 方法的对象。

此接口的实现负责存储 ServletConfig 对象，以便此方法可以返回该对象。实现此接口的 GenericServlet 类已经这样做了。

return	初始化此 servlet 的 ServletConfig 对象
See also	init

英文文档：

getServletConfig

ServletConfig getServletConfig()

Returns a ServletConfig object, which contains initialization and startup parameters for this servlet. The ServletConfig object returned is the one passed to the init method.

Implementations of this interface are responsible for storing the ServletConfig object so that this method can return it. The GenericServlet class, which implements this interface, already does this.

Returns:

the ServletConfig object that initializes this servlet

See Also:

init(javax.servlet.ServletConfig)

public void service(ServletRequest req, ServletResponse res) throws ServletException, java.io.IOException
由 servlet 容器调用，以允许 servlet 响应某个请求。

此方法仅在 servlet 的 init() 方法成功完成之后调用。

应该为抛出或发送错误的 servlet 设置响应的状态代码。

servlet 通常运行在可同时处理多个请求的多线程 servlet 容器中。开发人员必须知道要同步对所有共享资源（比如文件、网络连接以及 servlet 的类和实例变量）的访问。有关 Java 中多线程编程的更多信息，可从 the Java tutorial on multi-threaded programming 中获得。

req	包含客户端请求的 ServletRequest 对象
res	包含 servlet 的响应的 ServletResponse 对象
Throws	ServletException: 如果发生妨碍 servlet 正常操作的异常
Throws	java.io.IOException: 如果发生输入或输出异常

英文文档：

service

void service(ServletRequest req,
             ServletResponse res)
             throws ServletException,
                    IOException

Called by the servlet container to allow the servlet to respond to a request.

This method is only called after the servlet's init() method has completed successfully.

The status code of the response always should be set for a servlet that throws or sends an error.

Servlets typically run inside multithreaded servlet containers that can handle multiple requests concurrently. Developers must be aware to synchronize access to any shared resources such as files, network connections, and as well as the servlet's class and instance variables. More information on multithreaded programming in Java is available in the Java tutorial on multi-threaded programming.

Parameters:

req - the ServletRequest object that contains the client's request

res - the ServletResponse object that contains the servlet's response

Throws:

ServletException - if an exception occurs that interferes with the servlet's normal operation

IOException - if an input or output exception occurs

public String getServletInfo()
返回有关 servlet 的信息，比如作者、版本和版权。

此方法返回的字符串应该是纯文本，不应该是任何种类的标记（比如 HTML、XML，等等）。

return

包含 servlet 信息的 String

英文文档：

getServletInfo

String getServletInfo()

Returns information about the servlet, such as author, version, and copyright.

The string that this method returns should be plain text and not markup of any kind (such as HTML, XML, etc.).

Returns:

a String containing servlet information

public void destroy()
由 servlet 容器调用，指示将从服务中取出该 servlet。此方法仅在 servlet 的 service 方法已退出或者在过了超时期之后调用一次。在调用此方法之后，servlet 容器不会再对此 servlet 调用 service 方法。

此方法为 servlet 提供了一个清除持有的所有资源（比如内存、文件句柄和线程）的机会，并确保任何持久状态都与内存中该 servlet 的当前状态保持同步。

英文文档：

destroy

void destroy()

Called by the servlet container to indicate to a servlet that the servlet is being taken out of service. This method is only called once all threads within the servlet's service method have exited or after a timeout period has passed. After the servlet container calls this method, it will not call the service method again on this servlet.

This method gives the servlet an opportunity to clean up any resources that are being held (for example, memory, file handles, threads) and make sure that any persistent state is synchronized with the servlet's current state in memory.

Copyright 2007 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.