使用future机制和对象锁实现SynchronizedExecutor

野性酷女 2022-02-27 13:28 74阅读 0赞

### 使用future机制和对象锁实现SynchronizedExecutor ###

*   *  future
     *  callable
     *  executor
     *  SynchronizedExecutor
     *  总结

## future ##

future代表一个任务的预执行结果，通过get方法获取执行结果

public interface Future<V> { 
    		V get() throws Exception;
    	}

## callable ##

callable代表一个要执行的任务，执行方法call，执行完成返回一个值

public interface Callable<V> { 
    		V call() throws Exception;
    	}

## executor ##

executor为执行器，通过执行器来执行任务并得到预执行结果对象future

public interface Executor<V> { 
    		Future<V> execute(Callable<V> callable);
    	}

## SynchronizedExecutor ##

使用synchronized关键字实现的执行器

public class SynchronizedExecutor<V> implements Executor<V> { 
    	
    	static class ExecutorThread<V> extends Thread{ 
    		public Object lock;
    		public V result;
    		public Callable<V> task;
    		public Exception e;
    		public boolean isDone = false;
    		public ExecutorThread(Callable<V> task,Object lock) { 
    			this.lock = lock;
    			this.task = task;
    		}
    		@Override
    		public void run() { 
    			try { 
    				result = task.call();
    			} catch (Exception e) { 
    				this.e = e;
    			}finally { 
    				synchronized (lock) { 
    					//需持有锁才能调用notify
    					this.isDone = true;
    					lock.notifyAll();
    					//此处的加锁只是为了获得锁
    				}
    			}
    			
    		}
    		
    	}
    
    	@Override
    	public Future<V> execute(Callable<V> callable) { 
    		Object lock = new Object();
    		ExecutorThread<V> thread = new ExecutorThread<V>(callable,lock);
    		thread.start();
    		Future<V> future = new Future<V>() { 
    
    			@Override
    			public V get() throws Exception { 
    				synchronized (lock) { 
    					//通过锁机制，使得未完成时，get方法陷入等待队列，让出cpu给别的线程，异步完成时再唤醒
    					//需持有锁才能调用wait
    					while(!thread.isDone) { 
    						lock.wait();
    					}
    					if(thread.e != null) { 
    						throw thread.e;
    					}
    					return thread.result;
    				}
    			}
    			
    		};
    
    		return future;
    	}
    
    }

## 总结 ##

优点:通过多线程执行并立即返回一个Future对象,而不等待任务，使得源线程继续执行，  
只有当源线程需要多线程执行结果，调用其get方法时，通过创建执行线程时创建的对象锁来阻塞线程直到任务执行完成  
当执行过程中如果有抛出异常,则先捕获该异常，在调用get执行结果时再抛出

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