Java多线程之 FutureTask:带有返回值的函数定义和调用方式

import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;
public class FutureTaskDemo {
	public static void test2() throws Execution{
	    // 基于 Lambda 的 Callable 接口，在new FutureTask中的Lambda表达式即是Callable接口的实现
		FutureTask<Integer> task = new FutureTask<>(() -> {
			int t = 0;
			for (int i = 0; i < 10; i++)
				t += i;
			return t;
		}); 
		
		// 使用Thread类执行task
		System.out.println("Start calling.");
		long t1 = System.nanoTime();
		new Thread(task).start();
		long result = task.get();
		long t2 = System.nanoTime();
		System.out.println("Finish calling.");
		System.out.printf("Result: %d, Time: %.3f ms.\n", result, (t2 - t1) / 1000000f);
	}
}	

public class FutureTest1 {
	public static void main(String[] args) {
		Task task = new Task();// 新建异步任务
		FutureTask<Integer> future = new FutureTask<Integer>(task) {
			// 异步任务执行完成，回调
			@Override
			protected void done() {
				try {
					System.out.println("future.done():" + get());
				} catch (InterruptedException e) {
					e.printStackTrace();
				} catch (ExecutionException e) {
					e.printStackTrace();
				}
			}
		};
		// 创建线程池（使用了预定义的配置）
		ExecutorService executor = Executors.newCachedThreadPool();
		executor.execute(future);
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e1) {
			e1.printStackTrace();
		}
		// 可以取消异步任务
		// future.cancel(true);
		try {
			// 阻塞，等待异步任务执行完毕-获取异步任务的返回值
			System.out.println("future.get():" + future.get());
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}
	// 异步任务
	static class Task implements Callable<Integer> {
		// 返回异步任务的执行结果
		@Override
		public Integer call() throws Exception {
			int i = 0;
			for (; i < 10; i++) {
				try {
					System.out.println(Thread.currentThread().getName() + "_"
							+ i);
					Thread.sleep(500);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
			return i;
		}
	}
}

private Map<String, Connection> connectionPool = new HashMap<String, Connection>();  
private ReentrantLock lock = new ReentrantLock();    
public Connection getConnection(String key){  
    try{  
        lock.lock();  
        if(connectionPool.containsKey(key)){  
            return connectionPool.get(key);  
        }  
        else{  
            //创建 Connection  
            Connection conn = createConnection();  
            connectionPool.put(key, conn);  
            return conn;  
        }  
    }  
    finally{  
        lock.unlock();  
    }  
}  
  
//创建Connection（根据业务需求，自定义Connection）  
private Connection createConnection(){  
    return null;  
}  

private ConcurrentHashMap<String,FutureTask<Connection>>connectionPool = new ConcurrentHashMap<String, FutureTask<Connection>>();  
  
public Connection getConnection(String key) throws Exception{  
    FutureTask<Connection>connectionTask=connectionPool.get(key);  
    if(connectionTask!=null){  
        return connectionTask.get();  
    }  
    else{  
        Callable<Connection> callable = new Callable<Connection>(){  
            @Override  
            public Connection call() throws Exception {  
                // TODO Auto-generated method stub  
                return createConnection();  
            }  
        };  
        FutureTask<Connection>newTask = new FutureTask<Connection>(callable);  
        connectionTask = connectionPool.putIfAbsent(key, newTask);  
        if(connectionTask==null){  
            connectionTask = newTask;  
            connectionTask.run();  
        }  
        return connectionTask.get();  
    }  
}  
  
//创建Connection（根据业务需求，自定义Connection）  
private Connection createConnection(){  
    return null;  
}  

public FutureTask(Runnable runnable, V result) {  
        this.callable = Executors.callable(runnable, result);  
        this.state = NEW;       // ensure visibility of callable  
 } 

//这个方法有一个参数 是否中断running  
     public boolean cancel(boolean mayInterruptIfRunning) {  
          /** 
          * 这个有点晕啊逻辑关系是 
          * 等价与 if(state!=new || !UNSAFE.compareAndSwapInt(this, stateOffset, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)) 
          * 这个意思是 如果state不是new 那么就退出方法，这时的任务任务坑是已经完成了 或是被取消了 或是被中断了 
          * 如果是state 是new 就设置state 为中断状态 或是取消状态 
          * 
          **/  
        if (!(state == NEW &&  
              UNSAFE.compareAndSwapInt(this, stateOffset, NEW,  
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))  
            return false;  
        try {    // in case call to interrupt throws exception  
            //如果是可中断 那么就 调用系统中断方法 然后把状态设置成INTERRUPTED  
            if (mayInterruptIfRunning) {  
                try {  
                    Thread t = runner;  
                    if (t != null)  
                        t.interrupt();  
                } finally { // final state  
                    UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);  
                }  
            }  
        } finally {  
            finishCompletion();  
        }  
        return true;  
    }