当线程池中线程出现异常会发生什么

旧城等待， 2021-06-22 15:38 465阅读 0赞

#### 线程遇到未处理的异常就结束了 ####

这个好理解，当线程出现未捕获异常的时候就执行不下去了，留给它的就是垃圾回收了。

#### 当线程池中线程频繁出现未捕获异常 ####

当线程池中线程频繁出现未捕获的异常，那线程的复用率就大大降低了，需要不断地创建新线程。  
做个实验：

public class ThreadExecutor { 
    
    	private ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 60, TimeUnit.SECONDS,
    			new ArrayBlockingQueue<>(200), new ThreadFactoryBuilder().setNameFormat("customThread %d").build());
    
    	@Test
    	public void test() { 
    		IntStream.rangeClosed(1, 5).forEach(i -> { 
    			try { 
    				Thread.sleep(100);
    			} catch (InterruptedException e) { 
    				e.printStackTrace();
    			}
    			threadPoolExecutor.execute(() -> { 
    					int j = 1/0;
    		});});
    	}
    }

新建一个只有一个线程的线程池，每隔0.1s提交一个任务，任务中是一个1/0的计算。

Exception in thread "customThread 0" java.lang.ArithmeticException: / by zero
    	at thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    	at java.lang.Thread.run(Thread.java:748)
    Exception in thread "customThread 1" java.lang.ArithmeticException: / by zero
    	at thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    	at java.lang.Thread.run(Thread.java:748)
    Exception in thread "customThread 2" java.lang.ArithmeticException: / by zero
    	at thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    	at java.lang.Thread.run(Thread.java:748)
    Exception in thread "customThread 3" java.lang.ArithmeticException: / by zero
    	at thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    	at java.lang.Thread.run(Thread.java:748)
    Exception in thread "customThread 4" java.lang.ArithmeticException: / by zero
    	at thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    	at java.lang.Thread.run(Thread.java:748)
    Exception in thread "customThread 5" java.lang.ArithmeticException: / by zero
    	at thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    	at java.lang.Thread.run(Thread.java:748)

可见每次执行的线程都不一样，之前的线程都没有复用。原因是因为出现了未捕获的异常。

*  我们把异常捕获试试：

customThread 0 / by zero
    customThread 0 / by zero
    customThread 0 / by zero
    customThread 0 / by zero
    customThread 0 / by zero

可见当异常捕获了，线程就可以复用了。

#### 问题来了，我们的代码中异常不可能全部捕获 ####

如果要捕获那些没被业务代码捕获的异常，可以设置Thread类的uncaughtExceptionHandler属性。这时使用ThreadFactoryBuilder会比较方便，ThreadFactoryBuilder是guava提供的ThreadFactory生成器。

new ThreadFactoryBuilder()
    .setNameFormat("customThread %d")
    .setUncaughtExceptionHandler((t, e) -> System.out.println(t.getName() + "发生异常" + e.getCause()))
    .build()

修改之后：

public class ThreadExecutor { 
    
    	private static ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 60, TimeUnit.SECONDS,
    			new ArrayBlockingQueue<>(200),
    			new ThreadFactoryBuilder()
    					.setNameFormat("customThread %d")
    					.setUncaughtExceptionHandler((t, e) -> System.out.println("UncaughtExceptionHandler捕获到：" + t.getName() + "发生异常" + e.getMessage()))
    					.build());
    
    	@Test
    	public void test() { 
    		IntStream.rangeClosed(1, 5).forEach(i -> { 
    			try { 
    				Thread.sleep(100);
    			} catch (InterruptedException e) { 
    				e.printStackTrace();
    			}
    
    			threadPoolExecutor.execute(() -> { 
    				System.out.println("线程" + Thread.currentThread().getName() + "执行");
    				int j = 1 / 0;
    			});
    		});
    	}
    }

线程customThread 0执行
    UncaughtExceptionHandler捕获到：customThread 0发生异常/ by zero
    线程customThread 1执行
    UncaughtExceptionHandler捕获到：customThread 1发生异常/ by zero
    线程customThread 2执行
    UncaughtExceptionHandler捕获到：customThread 2发生异常/ by zero
    线程customThread 3执行
    UncaughtExceptionHandler捕获到：customThread 3发生异常/ by zero
    线程customThread 4执行
    UncaughtExceptionHandler捕获到：customThread 4发生异常/ by zero

可见，结果并不是我们想象的那样，线程池中原有的线程没有复用！ 所以通过UncaughtExceptionHandler想将异常吞掉使线程复用这招貌似行不通。它只是做了一层异常的保底处理。

*  将excute改成submit试试

public class ThreadExecutor { 
    
    	private static ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 60, TimeUnit.SECONDS,
    			new ArrayBlockingQueue<>(200),
    			new ThreadFactoryBuilder()
    					.setNameFormat("customThread %d")
    					.setUncaughtExceptionHandler((t, e) -> System.out.println("UncaughtExceptionHandler捕获到：" + t.getName() + "发生异常" + e.getMessage()))
    					.build());
    
    	@Test
    	public void test() { 
    		IntStream.rangeClosed(1, 5).forEach(i -> { 
    			try { 
    				Thread.sleep(100);
    			} catch (InterruptedException e) { 
    				e.printStackTrace();
    			}
    
    			Future<?> future = threadPoolExecutor.submit(() -> { 
    				System.out.println("线程" + Thread.currentThread().getName() + "执行");
    				int j = 1 / 0;
    			});
    			try { 
    				future.get();
    			} catch (InterruptedException e) { 
    				e.printStackTrace();
    			} catch (ExecutionException e) { 
    				e.printStackTrace();
    			}
    		});
    	}
    }

线程customThread 0执行
    java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
    线程customThread 0执行
    java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
    线程customThread 0执行
    java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
    线程customThread 0执行
    java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
    线程customThread 0执行
    java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero

通过submit提交线程可以屏蔽线程中产生的异常，达到线程复用。当get()执行结果时异常才会抛出。  
原因是通过submit提交的线程，当发生异常时，会将异常保存，待future.get();时才会抛出。  
这是Futuretask的部分run()方法，看setException：

public void run() { 
            try { 
                Callable<V> c = callable;
                if (c != null && state == NEW) { 
                    V result;
                    boolean ran;
                    try { 
                        result = c.call();
                        ran = true;
                    } catch (Throwable ex) { 
                        result = null;
                        ran = false;
                        setException(ex);
                    }
                    if (ran)
                        set(result);
                }
            } 
        }
    
        protected void setException(Throwable t) { 
            if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { 
                outcome = t;
                UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
                finishCompletion();
            }
        }

将异常存在outcome对象中，没有抛出，再看get方法：

public V get() throws InterruptedException, ExecutionException { 
            int s = state;
            if (s <= COMPLETING)
                s = awaitDone(false, 0L);
            return report(s);
        }

private V report(int s) throws ExecutionException { 
            Object x = outcome;
            if (s == NORMAL)
                return (V)x;
            if (s >= CANCELLED)
                throw new CancellationException();
            throw new ExecutionException((Throwable)x);
        }

当outcome是异常时才抛出。

#### 总结 ####

1、线程池中线程中异常尽量手动捕获  
2、通过设置ThreadFactory的UncaughtExceptionHandler可以对未捕获的异常做保底处理，通过execute提交任务，线程依然会中断，而通过submit提交任务，可以获取线程执行结果，线程异常会在get执行结果时抛出。