一、状态

线程共有6个状态。

状态	名称
new(新建状态)	用new创建的线程处于新建状态，此时他和其他的Java对象一样，仅仅在堆中分配了内存，没有调用start方法。
runable(就绪状态)	调用start方法，线程处于就绪状态
blocked(阻塞状态)	因为某些原因放弃CPU（缺少资源：IO、锁），暂时停止运行,进入阻塞状态
waiting(等待状态)	线程中的对象的调用wait()时，JVM就会将线程放入到等待池中
timed_waiting(睡眠状态)	在指定的等待时间内等待线程的状态
terminated(终止状态)	线程已经执行完毕

源码如下：

public enum State {/*** Thread state for a thread which has not yet started.*/NEW,/*** Thread state for a runnable thread.  A thread in the runnable* state is executing in the Java virtual machine but it may* be waiting for other resources from the operating system* such as processor.*/RUNNABLE,/*** Thread state for a thread blocked waiting for a monitor lock.* A thread in the blocked state is waiting for a monitor lock* to enter a synchronized block/method or* reenter a synchronized block/method after calling* {@link Object#wait() Object.wait}.*/BLOCKED,/*** Thread state for a waiting thread.* A thread is in the waiting state due to calling one of the* following methods:* <ul>*   <li>{@link Object#wait() Object.wait} with no timeout</li>*   <li>{@link #join() Thread.join} with no timeout</li>*   <li>{@link LockSupport#park() LockSupport.park}</li>* </ul>** <p>A thread in the waiting state is waiting for another thread to* perform a particular action.** For example, a thread that has called <tt>Object.wait()</tt>* on an object is waiting for another thread to call* <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on* that object. A thread that has called <tt>Thread.join()</tt>* is waiting for a specified thread to terminate.*/WAITING,/*** Thread state for a waiting thread with a specified waiting time.* A thread is in the timed waiting state due to calling one of* the following methods with a specified positive waiting time:* <ul>*   <li>{@link #sleep Thread.sleep}</li>*   <li>{@link Object#wait(long) Object.wait} with timeout</li>*   <li>{@link #join(long) Thread.join} with timeout</li>*   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>*   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>* </ul>*/TIMED_WAITING,/*** Thread state for a terminated thread.* The thread has completed execution.*/TERMINATED;}

正常的线程只有四个状态：新建状态（NEW）、RUNNABLE（就绪状态）、终止状态（TERMINATED）。

非正常状态：阻塞状态（BLOCKED）、等待状态（WAITING）、睡眠状态（TIME WAITING）

二、方法

2.1 start()

启动操作系统的线程，进入就绪队列，等待CPU分配时间片，获取到时间片后执行线程的run方法。（start()只能执行一次）

源码如下：

/*** Causes this thread to begin execution; the Java Virtual Machine* calls the <code>run</code> method of this thread.* <p>* The result is that two threads are running concurrently: the* current thread (which returns from the call to the* <code>start</code> method) and the other thread (which executes its* <code>run</code> method).* <p>* It is never legal to start a thread more than once.* In particular, a thread may not be restarted once it has completed* execution.** @exception  IllegalThreadStateException  if the thread was already*               started.* @see        #run()* @see        #stop()*/public synchronized void start() {/*** This method is not invoked for the main method thread or "system"* group threads created/set up by the VM. Any new functionality added* to this method in the future may have to also be added to the VM.** A zero status value corresponds to state "NEW".*/if (threadStatus != 0)throw new IllegalThreadStateException();/* Notify the group that this thread is about to be started* so that it can be added to the group's list of threads* and the group's unstarted count can be decremented. */group.add(this);boolean started = false;try {start0();started = true;} finally {try {if (!started) {group.threadStartFailed(this);}} catch (Throwable ignore) {/* do nothing. If start0 threw a Throwable thenit will be passed up the call stack */}}}

2.2 run()

业务逻辑的实现方法。

直接调用run方法不会启动线程

调用start方法以后，JVM会自动的运行run方法

源码如下：

/*** If this thread was constructed using a separate* <code>Runnable</code> run object, then that* <code>Runnable</code> object's <code>run</code> method is called;* otherwise, this method does nothing and returns.* <p>* Subclasses of <code>Thread</code> should override this method.** @see     #start()* @see     #stop()* @see     #Thread(ThreadGroup, Runnable, String)*/@Overridepublic void run() {if (target != null) {target.run();}}

2.3 yield()

将运行中的线程便会就绪状态，让出cpu的执行权，并通知操作系统从就绪线程中选出线程运行（可选择原来执行yield的线程）

注意：不释放资源

源码如下:

/*** A hint to the scheduler that the current thread is willing to yield* its current use of a processor. The scheduler is free to ignore this* hint.** <p> Yield is a heuristic attempt to improve relative progression* between threads that would otherwise over-utilise a CPU. Its use* should be combined with detailed profiling and benchmarking to* ensure that it actually has the desired effect.** <p> It is rarely appropriate to use this method. It may be useful* for debugging or testing purposes, where it may help to reproduce* bugs due to race conditions. It may also be useful when designing* concurrency control constructs such as the ones in the* {@link java.util.concurrent.locks} package.*/public static native void yield();

2.4 join()

当有新的线程加入时，主线程会进入等待(waiting)状态，一直到调用join()方法的线程执行结束为止。

当不调用join方法时，等到主线程执行完，才执行其他线程。

Runnable runnable = new RunnableDemo(){@Overridepublic void run() {System.out.println("执行中....");}
};
Thread t1 = new Thread(runnable);
Thread t2 = new Thread(runnable);
t1.start();
t2.start();
System.out.println("执行完毕");

执行结果：

执行完毕
执行中....
执行中....

调用join方法时，先执行指定线程

Runnable runnable = new RunnableDemo(){@Overridepublic void run() {System.out.println("执行中....");}
};
Thread t1 = new Thread(runnable);
Thread t2 = new Thread(runnable);
t1.start();
t2.start();try{t1.join();t2.join();
}catch (Exception e){e.printStackTrace();
}
System.out.println("执行完毕");

执行结果：

执行中....
执行中....
执行完毕

三、状态与方法关系图

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多线程(三）——了解线程

目录

一、状态

二、方法

2.1 start()

2.2 run()

2.3 yield()

2.4 join()

三、状态与方法关系图

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