Java Thread线程异常监控

一、场景描述：单线程程序可以用try...catch捕获程序的异常，而在多线程程序的时候是无法使用try...catch捕获。

示例1：多线程发生异常，无法使用try...catch捕获问题

public class NoCaughtThread  implements Runnable{
    @Override
    public void run() {
        System.out.println(3 / 2);
        System.out.println(3 / 0);
        System.out.println(3 / 1);
    }
    
    public static void main(String[] args) {
        try {
            Thread thread = new Thread(new NoCaughtThread());
            thread.start();
        } catch (Exception e) {
            System.out.println("==Exception: " + e.getMessage());
        }
    }
}

运行结果：

1
Exception in thread "Thread-0" java.lang.ArithmeticException: / by zero
at threadtest.NoCaughtThread.run(NoCaughtThread.java:7)
at java.lang.Thread.run(Thread.java:724)

显然这并非程序设定异常捕获，此时try...catch无法捕获线程的异常。此时，如果线程因为异常而终止执行将无法检测到异常。究其原因Thread类run()方法是不抛出任何检查型异常的，而自身却可能因为一个异常而被中止。

二、解决方式大致有两种：① 在run()中设置对应的异常处理，主动方法来解决未检测异常；② Thread类API中提供Interface接口UncaughtExceptionHandler，该接口包含uncaughtException方法，它能检测出某个未捕获的异常而终结的情况；

示例2：主动的检测异常

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class InitiativeCaught {
    public static void main(String[] args) {
        InitialtiveThread initialtiveThread = new InitialtiveThread() ;
        ExecutorService exec = Executors.newCachedThreadPool();
        exec.execute(initialtiveThread);
        exec.shutdown();
    }
}

class InitialtiveThread implements Runnable {
    @Override
    public void run() {
        Throwable thrown = null;
        try {
            System.out.println(3 / 2);
            System.out.println(3 / 0);
            System.out.println(3 / 1);
        } catch (Throwable e) {
            thrown = e;
        } finally {
            threadDeal(this, thrown);
        }
    }
    
    public void threadDeal(Runnable r, Throwable t) {
        System.out.println("==Exception: " + t.getMessage());
    }  
}

运行结果：

1
==Exception: / by zero

此时是主动捕获异常并做处理，得到想要的结果。

示例3：Thread类API中提供UncaughtExceptionHandler接口捕获异常，要求检测线程异常，发生异常设置为重复调用三次之后结束线程。

import java.lang.Thread.UncaughtExceptionHandler;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ThreadMonitor implements Runnable {
    private int data;                    // 可设置通过构造传参
    private int control = 0;
    private static final int MAX = 3;    // 设置重试次数
    
    public ThreadMonitor(int i) {
        this.data = i;    
    }
    public ThreadMonitor() {
        // TODO Auto-generated constructor stub
    }
    
    @Override
    public void run() {
        Thread.setDefaultUncaughtExceptionHandler(new UncaughtExceptionHandler() {
            @Override
            public void uncaughtException(Thread arg0, Throwable e) {
                // TODO Auto-generated method stub
                System.out.println("==Exception: " + e.getMessage());
                String message = e.getMessage();
                if( control==MAX ){
                    return ;
                }else if( "ok".equals(message) ){
                    return ;
                }else if ( "error".equals(message) ) {
                    new Thread() {
                        public void run() {
                            try {
                                System.out.println("开始睡眠。");
                                Thread.sleep(1 * 1000);
                                control++ ;
                                System.out.println("睡眠结束，control: "+ control);
                                myTask(data) ;
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        };
                    }.start();
                }else{
                    return ;
                }
            }
        });  
        
        myTask(data) ;
        
    }
    
    @SuppressWarnings("finally")
    public void myTask(int data){
        boolean flag = true ;
        try {
            System.out.println(4 / data);
        } catch (Exception e) {
            flag = false ;
        } finally {
            if( flag ){
                throw new RuntimeException("ok");
            }else{
                throw new RuntimeException("error");
            }
        }
    }
    
    public static void main(String[] args) {
        ExecutorService exec = Executors.newCachedThreadPool();
        ThreadMonitor threadMonitor = new ThreadMonitor(0);
        exec.execute(threadMonitor);
        exec.shutdown();
    }
}

运行结果：

==Exception: error
开始睡眠。
睡眠结束，control: 1
==Exception: error
开始睡眠。
睡眠结束，control: 2
==Exception: error
开始睡眠。
睡眠结束，control: 3
==Exception: error

此时，可以正常捕获线程因除数为零造成的中断。其中：

(1) 在Thread类API中提供Interface接口UncaughtExceptionHandler，该接口包含一个uncaughtException方法，它能检测出某个由于未捕获的异常而终结的情况。定义如下：

UncaughtExceptionHandler接口: public static interface Thread.UncaughtExceptionHandler

uncaughtException方法： public void uncaughtException(Thread t, Throwable e)

(2) uncaughtException方法会捕获线程的异常，此时需要覆写该方法设定自定义的处理方式。

(3) 设置UncaughtExceptionHandler异常处理：

方式一：通过Thread提供的静态static方法，设置默认异常处理：public static void setDefaultUncaughtExceptionHandler(Thread.UncaughtExceptionHandler ux)

方式二：通过方法：public void setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler eh)

(4) UncaughtExceptionHandler异常处理需要设置在run()方法内，否则无法捕获到线程的异常。

Java Thread线程异常监控

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