JAVA 创建线程的三种方式对比和解释

JAVA创建线程一共有三种方式，Thread类，Runable接口，Callable接口。

1)继承Thread类，Thread实现了Runable方法，调用start()方法，执行实现的run()方法。public class ThreadTest extends Thread {
private String name;

public ThreadTest(String name) {
this.name=name;
}

@Override
public void run() {
// TODO Auto-generated method stub
for(int i=0;i<10;i++) {
System.out.println(Thread.currentThread().getName()+name);
try {
Thread.sleep(new Random().nextInt(100));
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println(name+"finsh");
}

public static void main(String[] args) {
new ThreadTest("A").start();
new ThreadTest("B").start();
}

}执行结果：


2）通过实现Runnable接口，步骤1）实现runable接口，并实现run方法 2）创建Runable实例，通过Thread对象创建线程，调用start()方法import java.util.Random;

public class RunableTest implements Runnable{
private String name;
private int j;
public RunableTest(String name) {
// TODO Auto-generated constructor stub
this.name=name;
}
@Override
public void run() {
// TODO Auto-generated method stub
for(int i=0;i<10;i++) {

try {
Thread.sleep(new Random().nextInt(100));
j++;
System.out.println(Thread.currentThread().getName()+name+".."+j);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println(name+" finsh");
}

public static void main(String[] args) {
RunableTest R=new RunableTest("A");
new Thread(R).start();
new Thread(R).start();
}
}执行结果：

红色表示多个线程可以共享变量j，而Thread无法实现。

3)通过Callable和Future创建线程（1）创建Callable接口的实现类，并实现call()方法，该call()方法将作为线程执行体，并且有返回值。（2）创建Callable实现类的实例，使用FutureTask类来包装Callable对象，该FutureTask对象封装了该Callable对象的call()方法的返回值。（3）使用FutureTask对象作为Thread对象的target创建并启动新线程。（4）调用FutureTask对象的get()方法来获得子线程执行结束后的返回值import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;

public class CallableTest implements Callable<Integer> {

public String name;

public CallableTest(String name) {
// TODO Auto-generated constructor stub
this.name=name;
}
@Override
public Integer call() throws Exception {
// TODO Auto-generated method stub
int i=0;
for(;i<10;i++) {
System.out.println(Thread.currentThread().getName()+name);
try {
Thread.sleep(new Random().nextInt(100));
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println("A finsh");
return i;
}

public static void main(String[] args) {
CallableTest b=new CallableTest("A");
CallableTest c=new CallableTest("B");
FutureTask<Integer> Task1 = new FutureTask<Integer>(b);
FutureTask<Integer> Task2 = new FutureTask<Integer>(c);
new Thread(Task1).start();
new Thread(Task2).start();
}
}执行结果：


一个在论坛看到的问题：import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;

public class Test {

public static void main(String[] args) {
Thr hh=new Thr();
FutureTask<String> a=new FutureTask<String>(hh);
new Thread(a,"aaa").start();//开启线程aaa
System.out.println(1);
new Thread(a,"bbb").start();//开启线程bbb
System.out.println(2);
new Thread(a,"ccc").start();//开启线程ccc
System.out.println(3);

}
}

class Thr implements Callable<String>{
int i=0;
@Override
public String call() throws Exception {
/*
* 如果线程是aaa，则执行以下代码
*/
if(Thread.currentThread().getName().equals("aaa")){
for(;i<20;i++){
System.out.println("aaa"+i);
}
System.out.println("aaa finish");
return "aaa finish";
}

/*
* 如果线程是bbb，则执行以下代码
*/
else if(Thread.currentThread().getName().equals("bbb")){
for(;i<40;i++){
System.out.println("bbb"+i);
}
System.out.println("bbb finish");
return "bbb finish";
}

/*
* 如果线程是ccc，则执行以下代码
*/
else if(Thread.currentThread().getName().equals("ccc")){
for(;i<60;i++){
System.out.println("ccc"+i);
}
System.out.println("ccc finish");
return "ccc finish";
}
return "exe failure";

}

}这个执行结果为什么bb cc 不执行。 根本原因是当bbb和ccc执行调用new Thread(a,"bbb").start(); new Thread(a,"cc").start();这两个代码时 FutureTask 对象a 的状态已经不是NEW,见下面源码

public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW;       // ensure visibility of callable 初始化时state为NEW
}

FutureTask 实现了 RunnableFuture<V> 接口 ，RunnableFuture extends Runnable, Future<V>  。所以也会执行run()方法。

读下面前两句可以发现state!=NEW时会直接返回，而三个Thread共享一个 FutureTask a 对象 所以

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();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);//执行setResult会修改state值为其他值，见下图断点
}
} 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);
}
}

state更改后，因为启动的是一个对象，所以后两个启动线
4000
程，当执行到run方法是由于state不是0所以不会执行！执行结果也就合理了！
有问题欢迎指正谢谢！