java线程的创建有两种方式，这里我们通过简单的实例来学习一下。一切都明明白白，但我们仍匆匆错过，因为你相信命运，因为我怀疑生活。
java中多线程的创建
一、通过继承Thread类来创建多线程
public class HelloThread extends Thread {
@Override
public void run() {
try {
TimeUnit.SECONDS.sleep(1);
System.out.println("Hello from a thread!");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) throws InterruptedException {
HelloThread helloThread = new HelloThread();
helloThread.start();
System.out.println("In main thread.");
}
}
运行的结果如下：
In main thread.
Hello from a thread!
我们这里对Thread类的start的方法做一些说明，官方文档的说明：
Causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.
The result is that two threads are running concurrently: the current thread (which returns from the call to the start method) and the other thread (which executes its run method).
It is never legal(合法) to start a thread more than once(不止一次). In particular, a thread may not be restarted once it has completed execution. Throws:
IllegalThreadStateException - if the thread was already started
我们在上述例子的基础上，对上述的说明做一个代码的测试。在helloThread.start();代码的后面添加代码：
System.out.println(helloThread.getName());
helloThread.start();
一次的运行结果如下所示，在添加代码的第二行(helloThread.start();)报的错误。
Exception in thread "main" java.lang.IllegalThreadStateException
Thread-
at java.lang.Thread.start(Thread.java:)
at com.linux.huhx.thread.HelloThread.main(HelloThread.java:)
Hello from a thread!
如果在添加helloThread.start();之前让主线程睡眠3秒，也就是让先启动的helloThread线程执行完毕。我们再调用helloThread.start()启动线程。
TimeUnit.SECONDS.sleep();
System.out.println(helloThread.getName());
helloThread.start();
运行的结果如下：
Hello from a thread!
Thread-
Exception in thread "main" java.lang.IllegalThreadStateException
at java.lang.Thread.start(Thread.java:)
at com.linux.huhx.thread.HelloThread.main(HelloThread.java:)
二、通过实现Runnable接口来创建多线程
public class HelloRunnable implements Runnable {
@Override
public void run() {
try {
System.out.println("Hello from a thread!");
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Thread thread = new Thread(new HelloRunnable());
thread.start();
System.out.println("In main method.");
}
}
运行的结果如下：
In main method.
Hello from a thread!
这种使用实现HelloRunnable接口的方式是比较推崇的，因为java类中只能单继承可以多实现。现在看一下它的简单原理new Thread(new HelloRunnable())的源码如下：
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
init的方法是初始化线程的一些信息：
private void init(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc, boolean inheritThreadLocals) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
if (security != null) {
g = security.getThreadGroup();
}
if (g == null) {
g = parent.getThreadGroup();
}
}
g.checkAccess();
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
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;
setPriority(priority);
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
this.stackSize = stackSize;
tid = nextThreadID();
}
最重要的一行代码就是this.target = target;设置了线程target为我们的Runnable对象。我们都知道thread.start()启动一个线程，实际是调用线程的run方法。现在我们看一下Thread类的run方法。
@Override
public void run() {
if (target != null) {
target.run();
}
}
可以看到是调用了target的run方法，在我们这里就是(HelloRunnable)。对于继承Thread的的情况来说，就拿上述的HelloThread来讲，它里面的target是空的。不过执行的run方法是HelloThread重写的run方法。所以不存在HelloThread中没有target就不会执行的情况。