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(); }
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(); } }
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);
}
}
原文：http://www.cnblogs.com/MrZhang1/p/7470343.html