探究Tomcat中JSP热部署机制

<servlet>
	<servlet-name>jsp</servlet-name>
	<servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
	<init-param>
		<param-name>fork</param-name>
		<param-value>false</param-value>
	</init-param>
	<init-param>
		<param-name>xpoweredBy</param-name>
		<param-value>false</param-value>
	</init-param>
	<load-on-startup>3</load-on-startup>
</servlet>

<servlet-mapping>
	<servlet-name>jsp</servlet-name>
	<url-pattern>*.jsp</url-pattern>
</servlet-mapping>

public void service(HttpServletRequest request, HttpServletResponse response){
	
	.......
	//判断你请求jsp页面时有没有带?jsp_precompile查询字符串，如果带了就会重新编译
	boolean precompile = preCompile(request);
	/*判断jsp是否要进行编译处理,并通过jspUri获取JspServletWrapper对象
	 *若JspServletWrapper对象为空,则判断jspUrl对应的JSP文件是否存在
	 *／
	serviceJspFile(request,response,jspUri,precompile);
	.......
}

private void serviceJspFile(HttpServletRequest request,
			HttpServletResponse response, String jspUri,
			boolean precompile){
	......
	JspServletWrapper wrapper = rctxt.getWrapper(jspUri);
	if(wrapper == null){
		synchronized(this){
			wrapper = rctxt.getWrapper(jspUri);
			......
			//判断jspUri对应的资源是否存在
			handleMissingResource(request, response, jspUri);
			......
		}
	}

	......
	wrapper.service(request, response, precompile);
	......
}

public void service(HttpServletRequest request,
		  HttpServletResponse response, boolean procompile){
	
	
	/*
	 * 校验逻辑(判断jsp是否可以使用)
	 */		
	......

	//Compile编译
	......
		//设置reload属性为true;即重新加载servlet
		ctxt.compile();
	......

	//加载Jsp文件对应的Servlet
	servlet = getServlet();
	......
}	

public void compile(){
	//创建编译器
	createCompile();
	
	if(jspCompiler.isOutDated()){
		//JSP文件不存在则抛出异常
		if(isRemove())
			throw new FileNotFoundException(jspUri);
		......
		//进行编译
		jspCompiler.compiler();
		//设置reload属性true,表明jsp文件修改,需要重新加载
		jsw.setReload(true);
		......
	}		
}

public boolean isOutDated(boolean checkClass){
	
	if (jsw != null
            		&& (ctxt.getOptions().getModificationTestInterval() > 0)) {
	     
                    /*
                     * 4秒缓存机制，其中ctxt.getOptions()得到的是EmbeddedServletOptions
	       * 类,这个类中定义有modificationTestInterval = 4
	       * 如果jsp文件修改了,需等到4秒之后才会重新编译加载;
                     */
	  	      if (jsw.getLastModificationTest()
              		  + (ctxt.getOptions().getModificationTestInterval() * 1000) 			  > System.currentTimeMillis()) {
            			return false;
        	       }
        	      jsw.setLastModificationTest(System.currentTimeMillis());
 	         }
	
	File targetFile;
	//拿到工作目录下的class文件对象
    		if (checkClass) {
       	 	targetFile = new File(ctxt.getClassFileName());
    		} else {
        		targetFile = new File(ctxt.getServletJavaFileName());
    		}
    		if (!targetFile.exists()) {
        		return true;
    		}
    		long targetLastModified = targetFile.lastModified();
    		if (checkClass && jsw != null) {
        		jsw.setServletClassLastModifiedTime(targetLastModified);
   		}
	
	/*
	 * 判断jsp文件的修改时间,实际处理就是将jsp文件的时间与class文件时间进行对比比较
	 */
    		Long jspRealLastModified = ctxt.getLastModified(ctxt.getJspFile());
    		if (jspRealLastModified.longValue() < 0) {
        		// Something went wrong - assume modification
        		return true;
    		}
	
	//比较JSP文件与class文件的时间戳
    		if (targetLastModified != jspRealLastModified.longValue()) {
        		if (log.isDebugEnabled()) {
            			log.debug("Compiler: outdated: " + targetFile + " 							"+ targetLastModified);
       		}
        		return true;
    		}
}