Java多线程

Posted 2020-10-05 mrzhang1

Java中的多线程概述

Java中的线程都是通过Thread类来管理的。Thread类中维护一个target，该target就是用户要创建的任务对象。所以要创建多任务必须实现Runnable接口。当然继承Thread类也可以实现多任务。

 private Runnable target;
private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc) {
......................
        g.addUnstarted();

        this.group = g;
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;  //target 也就是runnable接口类型的对象。
        setPriority(priority);
        if (parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

　　

第一种方式：

实现runnable接口，并实现run（）方法。 该方式任务没有返回值

class demo1 implements Runnable {
    
    @Override
    public void run() {
        while( true ){
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            show1();
        }
    }

    private synchronized void show1() {
        System.out.println("demo1_show1");
    }
}

class test{
    
    public static void main(String[] args) {
                
        Thread t1 = new Thread(new demo1());        
        t1.start();

    }
}

第二种方式：

实现Callable<V>接口，FutureTask<V>实现了Runnable接口的run()方法，该方法中调用的Callbale<V>中的call()方法，并保存了返回值。所以这里的实现方式是多封装了一层。

public class CallableDemo implements Callable<Integer>{

	@Override
	public Integer call() throws Exception {
		// TODO Auto-generated method stub
		System.out.println("call");
		Thread.sleep(100);
		return 1;
	}

	@Test
	public void  init( ) throws InterruptedException, ExecutionException{		
		FutureTask<Integer> futureTask = new FutureTask<>(new CallableDemo());
		new Thread(futureTask).start();
		System.out.println("taskRet ： "+futureTask.get());
	}
}

　因为最后都是通过Thread类管理的线程。而Callable没有继承Runnable接口。但是FutureTask<V>实现了Runable接口，并实现了run()方法。FutureTask<V>的run()方法源码如下：

public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();  //获取call（）函数的返回值。
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);  //保存返回值，通过get（）方法获取该返回值。
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }