从一道题目验证Servlet单实例

冷不防 2022-08-19 13:19 184阅读 0赞

# 1、题目 #

在servlet的开发中，人们通常喜欢用StringBuffer对象来append日志，然后将日志打印出来。在走读某同事的代码时，发现其servlet代码是这么写的

public class DoSomethingServlet extends HttpServlet {
    	private static Logger logger = org.apache.log4j.Logger.getLogger(DoSomethingServlet.class);
        private StringBuffer sb = new StringBuffer();
        
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            sb.append("日志信息");  
            //...  
            logger.info(sb.toString());
    	}
    
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    }

请问这位同事的代码有什么问题?

# 2、初探 #

我想这大概是和线程安全有关吧，多线程访问下日志会互窜？动手做个试验，首先是servlet

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		int userid = Integer.valueOf(request.getParameter("userid"));  
    		sb.append("userid="+userid+" ");  
    		logger.info(sb.toString());  
    		PrintWriter out = response.getWriter();  
    		out.print("process result！"); 
    	}

由于用浏览器模拟多个用户访问比较麻烦，下面通过代码发送http请求

import java.io.ByteArrayOutputStream;
    import java.io.InputStream;
    import java.io.OutputStream;
    import java.net.HttpURLConnection;
    import java.net.URL;
    
    
    public class UserThread extends Thread{
    
    	private int userid;
    
    	public UserThread(int userid) {
    		super();
    		this.userid = userid;
    	}
    
    	@Override
    	public void run() {
    		super.run();
    		try {
    			request();
    		} catch (Exception e) {
    			e.printStackTrace();
    		}
    	};
    	private String request() throws Exception{
    		String url = "http://localhost:8080/ServletSingleThread/DoSomething?userid="+userid;
    		System.out.println(url);
    		URL u = new URL(url);
    	    HttpURLConnection con;
    	    InputStream in;
    	    OutputStream out;
    	    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    	    byte[] data =new byte[4096];
    	    con = (HttpURLConnection) u.openConnection();     
    	    con.setRequestMethod("GET");
    	    con.setDoInput(true);
    	    con.setDoOutput(true);
    		
    	    out = con.getOutputStream();
    	    out.write(0);
    	    out.flush();
    	    
    	    in = con.getInputStream();
    	    int i=0;
    	    while((i=in.read(data))!=-1){
    	    	baos.write(data, 0, i);
    	    }
    	    out.close();
    	    in.close();
    	    baos.close();
    		return baos.toString();
    	}
    	public static void main(String[] args) {
    		//模拟多用户请求
    		for (int i = 0; i < 10; i++) {
    			UserThread ut = new UserThread(i);	
    			ut.start();
    		}
    	}
    }

log4j的配置就不贴上来，将日志输出到控制台，启动服务器，运行UserThread，观察日志：

[INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 userid=8 userid=3 userid=2 userid=9 userid=0 userid=1 userid=4 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 userid=8 userid=3 userid=2 userid=9 userid=0 userid=1 
    [INFO] 2016-01-16 16:18:26 : userid=6 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 userid=8 userid=3 userid=2 userid=9 userid=0 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 userid=8 userid=3 userid=2 userid=9 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 userid=8 userid=3 userid=2 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 userid=8 userid=3 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 
    [INFO] 2016-01-16 16:18:26 : userid=6 userid=7 userid=5 userid=8

这并不是我们想要的日志，不同用户的请求日志窜到了一起，但是仔细看会发现日志还是有规律的，userid被按线程执行顺序添加到了同一个StringBuffer中，说明了对于多个用户的请求，共用了同一个Servlet，可见Servlet是单例的。所有对于题目的问题，可以这么解决：将StringBuffer设置成局部变量。

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		int userid = Integer.valueOf(request.getParameter("userid"));  
    		StringBuffer sb = new StringBuffer();
    		sb.append("userid="+userid+" ");  
    		logger.info(sb.toString());  
    		PrintWriter out = response.getWriter();  
    		out.print("process result！"); 
    	}

再跑一遍，观察日志：

[INFO] 2016-01-16 16:33:47 : userid=2 
    [INFO] 2016-01-16 16:33:47 : userid=0 
    [INFO] 2016-01-16 16:33:47 : userid=4 
    [INFO] 2016-01-16 16:33:47 : userid=7 
    [INFO] 2016-01-16 16:33:47 : userid=6 
    [INFO] 2016-01-16 16:33:47 : userid=5 
    [INFO] 2016-01-16 16:33:47 : userid=1 
    [INFO] 2016-01-16 16:33:47 : userid=9 
    [INFO] 2016-01-16 16:33:47 : userid=3 
    [INFO] 2016-01-16 16:33:47 : userid=8

这才是我们想要的正常的日志 ，每个线程在自己的堆栈空间执行方法，StringBuffer在该方法体中，所有不会在多线程中共享。

# 3、深究 #

在刚刚的测试中，并没有很好的体现多线程并发访问问题，有人可能会想，这样写也可以解决题目的问题

synchronized(this){
        		sb.append("userid="+userid+" ");
        	}

测试结果和第一种情况一样，其实append本身就是一个同步方法，在这里加个同步块并没有意义。那如果是StringBuilder呢？

public class DoSomethingServlet extends HttpServlet {
    	private static Logger logger = org.apache.log4j.Logger.getLogger(DoSomethingServlet.class);
    	private StringBuilder sb = new StringBuilder();
        
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		int userid = Integer.valueOf(request.getParameter("userid"));  
    		sb.append("userid="+userid+" ");  
    		logger.info(sb.toString());  
    		PrintWriter out = response.getWriter();  
    		out.print("process result！"); 
    	}
    
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    }

测试结果：

[INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 userid=5 userid=6 userid=8 userid=2 userid=4 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 userid=5 userid=6 userid=8 userid=2 userid=4 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 userid=5 userid=6 userid=8 userid=2 userid=4 userid=9 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 userid=5 userid=6 userid=8 userid=2 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 userid=5 userid=6 userid=8 
    [INFO] 2016-01-16 16:56:43 : userid=7 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 userid=5 
    [INFO] 2016-01-16 16:56:43 : userid=7 userid=3 userid=1 userid=5 userid=6

仔细观察会发现和第一种情况是不一样的，1、2行日志相同了。因为StringBuilder是线程不安全的，多个线程并发append时就会出现数据不一致：在1、2行日志输出之前，userid=0被userid=4覆盖了。更直观点可以有String来测试：

public class DoSomethingServlet extends HttpServlet {
           
    	private static Logger logger = org.apache.log4j.Logger.getLogger(DoSomethingServlet.class);
        private String str;
        
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            int userid = Integer.valueOf(request.getParameter("userid"));
        	str = "userid="+userid+" ";
    		logger.info(str);
    		PrintWriter out = response.getWriter();
    		out.print("process result！");
    	}
    
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    }

测试结果：

[INFO] 2016-01-16 17:14:15 : userid=1 
    [INFO] 2016-01-16 17:14:15 : userid=6 
    [INFO] 2016-01-16 17:14:15 : userid=4 
    [INFO] 2016-01-16 17:14:15 : userid=9 
    [INFO] 2016-01-16 17:14:15 : userid=3 
    [INFO] 2016-01-16 17:14:15 : userid=5 
    [INFO] 2016-01-16 17:14:15 : userid=9 
    [INFO] 2016-01-16 17:14:15 : userid=2 
    [INFO] 2016-01-16 17:14:15 : userid=8 
    [INFO] 2016-01-16 17:14:15 : userid=0

userid为7的用户的日志被userid为9的用户对应的处理线程给改了

# 4、结论 #

这道题目的解释就是，由于Servlet是单例的，所以所有请求共享同一个成员变量StringBuffer，但是由于StringBuffer是线程安全的，所有StringBuffer的值并不会因为并发访问而相互覆盖，只是不断的新增。如果使用StringBuffer来存储日志信息，请将其定义为局部变量。