Servlet源码解读

Servlet源码解读

此源码解读是基于servlet-api-2.5.

Servlet接口

Servlet接口是servlet包中最核心的接口,是所有Servlet必须直接或间接实现的接口。

public interface Servlet{
    publi c void init(ServletConfig config) throws ServletException;
    public ServletConfig getServletConfig();
    public void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException;
    public String getServletInfo();
    public void destroy();
}

每一次新建servlet，都要实现Servlet接口里的方法，为了减轻我们的负担，Servlet包里为我们提供了一个模板类GenericServlet。该抽象类继承自Servlet，并实现了若干通用的初始化过程。1

public abstract class GenericServlet 
    implements Servlet, ServletConfig, java.io.Serializable{}

抽象类GenericServlet中有一个成员变量ServletConfig:

private transient ServletConfig config;

该成员变量封装了Servlet的相关配置信息。初始化时，Servlet容器调用Servlet的init()方法，将读取到的web.xml中的对应的Servlet配置信息封装成ServletConfig作为参数，传递给init()函数。

/**
     *
     * Called by the servlet container to indicate to a servlet that the
     * servlet is being placed into service.  See {@link Servlet#init}.
     *
     * <p>This implementation stores the {@link ServletConfig}
     * object it receives from the servlet container for later use.
     * When overriding this form of the method, call 
     * <code>super.init(config)</code>.
     *
     * @param config            the <code>ServletConfig</code> object
     *                  that contains configutation
     *                  information for this servlet
     *
     * @exception ServletException  if an exception occurs that
     *                  interrupts the servlet's normal
     *                  operation
     *
     * 
     * @see                 UnavailableException
     *
     */

    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        this.init();
    }

同时，GenericServlet也实现了Servlet接口中定义的getServletConfig()方法，用以实现对ServletConfig config成员变量的访问。

ServletRequest和ServletRespouse接口

Servlet包中另外两个重要的接口是ServletRequest和ServletResponse。对于每个HTTP请求，Servlet容器都会封装一个ServletRequest对象传递给Servlet的service()方法,通过ServletRequest,可以获得一些请求相关的参数.ServletResponse则封装了一个Servlet的响应，通过ServletResponse可以设置一些参数和内容，进行返回。不过在返回之前，为了避免出现问题，通常会调用setContentType()方法，设置返回内容的类型，如果不设置，会默认以html的形式返回。

/**
     * Sets the content type of the response being sent to
     * the client, if the response has not been committed yet.
     * The given content type may include a character encoding
     * specification, for example, <code>text/html;charset=UTF-8</code>.
     * The response's character encoding is only set from the given
     * content type if this method is called before <code>getWriter</code>
     * is called.
     * <p>This method may be called repeatedly to change content type and
     * character encoding.
     * This method has no effect if called after the response
     * has been committed. It does not set the response's character
     * encoding if it is called after <code>getWriter</code>
     * has been called or after the response has been committed.
     * <p>Containers must communicate the content type and the character
     * encoding used for the servlet response's writer to the client if
     * the protocol provides a way for doing so. In the case of HTTP,
     * the <code>Content-Type</code> header is used.
     *
     * @param type  a <code>String</code> specifying the MIME 
     *          type of the content
     *
     * @see         #setLocale
     * @see         #setCharacterEncoding
     * @see         #getOutputStream
     * @see         #getWriter
     *
     */

    public void setContentType(String type);

ServletResponse中还定义了getWriter()方法，将文本返回给客户端的java.io.PrintWriter,在默认的情况下，PrintWriter对象使用ISO-8859-1编码,这可能会引起乱码.

    /**
     * Returns a <code>PrintWriter</code> object that
     * can send character text to the client.
     * The <code>PrintWriter</code> uses the character
     * encoding returned by {@link #getCharacterEncoding}.
     * If the response's character encoding has not been
     * specified as described in <code>getCharacterEncoding</code>
     * (i.e., the method just returns the default value 
     * <code>ISO-8859-1</code>), <code>getWriter</code>
     * updates it to <code>ISO-8859-1</code>.
     * <p>Calling flush() on the <code>PrintWriter</code>
     * commits the response.
     * <p>Either this method or {@link #getOutputStream} may be called
     * to write the body, not both.
     *
     * 
     * @return      a <code>PrintWriter</code> object that 
     *          can return character data to the client 
     *
     * @exception UnsupportedEncodingException
     *          if the character encoding returned
     *          by <code>getCharacterEncoding</code> cannot be used
     *
     * @exception IllegalStateException
     *          if the <code>getOutputStream</code>
     *          method has already been called for this 
     *          response object
     *
     * @exception IOException
     *          if an input or output exception occurred
     *
     * @see         #getOutputStream
     * @see         #setCharacterEncoding
     *
     */

    public PrintWriter getWriter() throws IOException;

ServletConfig和ServletContext接口

ServletConfig如上文介绍的，封装了对应Servlet的配置信息。
ServletContext则是一个全局的储存信息的空间。服务启动时，便会加载ServletContext，服务关闭时，才会释放ServletContext。2

HttpServlet、HttpServletRequest和HttpServletResponse

下面会提到三个新的接口或抽象类，下面先看看这三个类与上文提到的类有什么关系.
HttpServletRequest继承自ServletRequest

public interface HttpServletRequest extends ServletRequest

HttpServletResponse继承自ServletResponse

public interface HttpServletResponse extends ServletResponse

HttpServlet继承自GenericServlet

public abstract class HttpServlet extends GenericServlet
    implements java.io.Serializable

HttpServlet重写了doGet()方法和doPost()方法，并且用HttpServletRequest和HttpServletResponse替换了ServletRequest和ServletResponse

HttpServletRequest继承自ServletRequest

public interface HttpServletRequest extends ServletRequest

HttpServletResponse继承自ServletResponse

public interface HttpServletResponse extends ServletResponse

总结

在servlet-api-2.5中，最主要的成员包括: Servlet、GenericServlet、ServletRequest、ServletResponse，以及在http包下的分别继承自以上若干类的HttpServlet、HttpServletRequest和HttpServletResponse。当然还包括以后提到的HttpSeesion和Cookie。

参考资料

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1. http://www.jianshu.com/p/e9f31c783ff1 ↩
2. http://blog.youkuaiyun.com/bryanliu1982/article/details/5214899 ↩