Tomcat主线程监听SHUTDOWN，如何远程关闭Tomcat？守护线程守护了谁？(1)

二、Tomcat主线程监听SHUTDOWN

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Tomcat中的线程也有两种，主线程是非守护线程，其他单独创建的线程或者线程池创建的工作线程都默认是守护线程。

当Tomcat启动时，做完所有初始化和启动工作，主线程会进入一个无限循环监听默认8005端口的状态，直到网络中读取到SHUTDOWN指令，才会退出循环，进而调用Tomcat停止销毁操作。

源码展示

org.apache.catalina.startup.Catalina#start

public void start() {

// 省略调用Server init、start等操作

if (await) {

// 调用Server的await，循环等待shutdown指令

await();

// 如果接收到shutdown，就结束await()，调用stop停止Tomcat

stop();

}

}

源码很简单，建立一个ServerSocket，循环监听读取网络中是否有SHUTDOWN指令传来：

public void await() {

// 省略部分无关紧要代码

// Set up a server socket to wait on

try {

// 建立一个server socket 端口默认为8005

awaitSocket = new ServerSocket(getPortWithOffset(), 1,

InetAddress.getByName(address));

} catch (IOException e) {

log.error(sm.getString(“standardServer.awaitSocket.fail”, address,

String.valueOf(getPortWithOffset()), String.valueOf(getPort()),

String.valueOf(getPortOffset())), e);

return;

}

try {

awaitThread = Thread.currentThread();

// Loop waiting for a connection and a valid command

while (!stopAwait) {

ServerSocket serverSocket = awaitSocket;

if (serverSocket == null) {

break;

}

// Wait for the next connection

Socket socket = null;

StringBuilder command = new StringBuilder();

try {

InputStream stream;

long acceptStartTime = System.currentTimeMillis();

try {

socket = serverSocket.accept();

socket.setSoTimeout(10 * 1000); // Ten seconds

stream = socket.getInputStream();

} catch (SocketTimeoutException ste) {

// This should never happen but bug 56684 suggests that

// it does.

log.warn(sm.getString(“standardServer.accept.timeout”,

Long.valueOf(System.currentTimeMillis() - acceptStartTime)), ste);

continue;

} catch (AccessControlException ace) {

log.warn(sm.getString(“standardServer.accept.security”), ace);

continue;

} catch (IOException e) {

if (stopAwait) {

// Wait was aborted with socket.close()

break;

}

log.error(sm.getString(“standardServer.accept.error”), e);

break;

}

// 读取 command

while (expected > 0) {

int ch = -1;

try {

ch = stream.read();

} catch (IOException e) {

log.warn(sm.getString(“standardServer.accept.readError”), e);

ch = -1;

}

// Control character or EOF (-1) terminates loop

if (ch < 32 || ch == 127) {

break;

}

command.append((char) ch);

expected–;

}

} finally {

// Close the socket now that we are done with it

try {

if (socket != null) {

socket.close();

}

} catch (IOException e) {

// Ignore

}

}

// Match against our command string

// 匹配shutdown指令

boolean match = command.toString().equals(shutdown);

if (match) {

log.info(sm.getString(“standardServer.shutdownViaPort”));

// 匹配成功，退出循环

break;

} else

log.warn(sm.getString(“standardServer.invalidShutdownCommand”, command.toString()));

}

} finally {

ServerSocket serverSocket = awaitSocket;

awaitThread = null;

awaitSocket = null;

// Close the server socket and return

if (serverSocket != null) {

try {

serverSocket.close();

} catch (IOException e) {

// Ignore

}

}

}

}

Tomcat的Connector是在线程池里处理请求连接，可以引用公共的Executor组件，也可以创建私有的线程池，线程池中创建的工作线程默认都是守护线程，这样web项目里创建的线程，默认也都是守护线程。Tomcat主线程退出，web项目中的用户线程也跟着退出。

如果用户线程被私自设置成非守护线程，或者设置Connector中的线程池创建的工作线程是非守护的，就会导致用户的非守护线程阻碍Tomcat主线程的正常退出。

三、模拟Tomcat远程关闭

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1、正常远程关闭

运行Tomcat，并保持默认SHUTDOWN端口8005。Tomcat启动成功后，运行如下代码即可向8005发送SHUTDOWN指令：

public class TestTomcatShutdown {

public static void main(String[] args) throws InterruptedException {

Socket socket = null;

try {

socket = new Socket(“127.0.0.1”, 8005);

String shutdown = “SHUTDOWN”;

socket.getOutputStream().write(shutdown.getBytes());

} catch (IOException e) {

e.printStackTrace();

} finally {

if (socket != null) {

try {

socket.close();

} catch (IOException e) {

e.printStackTrace();

}

}

}

}

}

从Tomcat日志中可以看出，Tomcat收到了一个合法的SHUTDOWN指令，进而调用了一些停止销毁操作。

2、非守护用户线程下远程关闭

如果用户创建的线程是非守护线程，看看Tomcat收到SHUTDOWN指令后能否正常退出。简单写一个Servlet并创建一个循环运行的非守护线程，部署到Tomcat中：

package com.stefan;

import jakarta.servlet.ServletException;

import jakarta.servlet.http.HttpServlet;

import jakarta.servlet.http.HttpServletRequest;

import jakarta.servlet.http.HttpServletResponse;

import java.io.IOException;

public class TestShutdownServlet extends HttpServlet {

@Override

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

System.out.println(“TestShutdownServlet doGet…”);

System.out.println(TestShutdownServlet.class.getClassLoader());

System.out.println(Thread.currentThread().getName());

TestThread testThread = new TestThread();

testThread.setDaemon(false);

testThread.start();

System.out.println(“testThread.isDaemon()=” + testThread.isDaemon());

}

}

package com.stefan;

public class TestThread extends Thread {

@Override

public void run() {

while (true) {

System.out.println(“hhhhhhhh”);

try {

Thread.sleep(3000);

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}

}

将上述两个类编译，并建立一个web目录test，放到webapps下即可：

在web.xml里指定servlet映射：

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns=“https://jakarta.ee/xml/ns/jakartaee”

xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance”

xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee

https://jakarta.ee/xml/ns/jakartaee/web-app_5_0.xsd"

version=“5.0”

metadata-complete=“true”>

test tomcat SHUTDOWN

test tomcat SHUTDOWN

UTF-8

TestShutdownServlet