java多线程2：Thread中的方法
静态方法：
Thread类中的静态方法表示操作的线程是"正在执行静态方法所在的代码块的线程"。
为什么Thread类中要有静态方法，这样就能对CPU当前正在运行的线程进行操作。下面来看一下Thread类中的静态方法：
1：currentThread
/** * Returns a reference to the currently executing thread object. * * @return the currently executing thread. */ public static native Thread currentThread();
currentThread()方法返回的是对当前正在执行线程对象的引用。
/**
* 测试Thread.currentThread()：返回代码段正在被哪个线程调用的信息
* this 始终表示线程实例本身
*
* @author yuxiaoyu
*/
public class Mythread4 extends Thread {
static {
System.out.println("Mythread4静态块的打印：Thread.currentThread().getName()=" + Thread.currentThread().getName());
}
public Mythread4() {
System.out.println("Mythread4构造方法：Thread.currentThread().getName()=" + Thread.currentThread().getName());
System.out.println("Mythread4构造方法：this.getName()=" + this.getName());
}
public void run() {
System.out.println("run 方法：Thread.currentThread().getName()=" + Thread.currentThread().getName());
System.out.println("run 方法：this.getName()=" + this.getName());
}
}
@Test
public void test4() {
Mythread4 a = new Mythread4();
a.start();
}
执行结果：
Mythread4静态块的打印：Thread.currentThread().getName()=main Mythread4构造方法：Thread.currentThread().getName()=main Mythread4构造方法：this.getName()=Thread-0 run 方法：Thread.currentThread().getName()=Thread-0 run 方法：this.getName()=Thread-0
这个结果说明，线程类的构造方法、静态块是被main线程调用的，而线程类的run()方法才是应用线程自己调用的，this 始终表示线程实例本身。
2：sleep
/**
* Causes the currently executing thread to sleep (temporarily cease
* execution) for the specified number of milliseconds, subject to
* the precision and accuracy of system timers and schedulers. The thread
* does not lose ownership of any monitors.
*
* @param millis
* the length of time to sleep in milliseconds
*
* @throws IllegalArgumentException
* if the value of {@code millis} is negative
*
* @throws InterruptedException
* if any thread has interrupted the current thread. The
* interrupted status of the current thread is
* cleared when this exception is thrown.
*/
public static native void sleep(long millis) throws InterruptedException;
该方法是让正在执行的线路休眠指定毫秒，注意API的注释中“The thread does not lose ownership of any monitors.”表示线程不会失去任何监视器的所有权。
简单说就是sleep代码上下文如果被加锁了，那么在休眠的时间内，锁依然在，但是CPU资源会让出给其他线程。
3：yield
/**
* 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.
*
* 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.
*
*
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();
该方法表示给调度程序的一个提示:当前线程愿意让出它当前对处理器的使用。调度程序可以自由地忽略这个提示。
public class Mythread5 extends Thread {
public void run() {
long beginTime = System.currentTimeMillis();
int count = 0;
for (int i = 0; i < 50000000; i++)
{
Thread.yield();
count = count + i + 1;
}
long endTime = System.currentTimeMillis();
System.out.println("用时：" + (endTime - beginTime) + "毫秒！");
}
}
第一次运行结果：用时：34872毫秒！
第二次运行结果：用时：33738毫秒！
如果将 Thread.yield(); 注释掉的话， 运行结果：用时：48毫秒！。
看到，每次执行的用时都不一样，证明了yield()方法放弃CPU的时间并不确定，而且yield方法将CPU资源让给其他资源导致变慢。
4：interrupted()
/**
* Tests whether the current thread has been interrupted. The
* interrupted status of the thread is cleared by this method. In
* other words, if this method were to be called twice in succession, the
* second call would return false (unless the current thread were
* interrupted again, after the first call had cleared its interrupted
* status and before the second call had examined it).
*
* A thread interruption ignored because a thread was not alive
* at the time of the interrupt will be reflected by this method
* returning false.
*
* @return true if the current thread has been interrupted;
* false otherwise.
* @see #isInterrupted()
* @revised 6.0
*/
public static boolean interrupted() {
return currentThread().isInterrupted(true);
}
/**
* Tests if some Thread has been interrupted. The interrupted state
* is reset or not based on the value of ClearInterrupted that is
* passed.
*/
private native boolean isInterrupted(boolean ClearInterrupted);
测试当前线程是否已经中断，并清除线程的中断状态。
实例方法
1：start
/**
* 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 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 then
it will be passed up the call stack */
}
}
}
此方法告诉线程执行器，这个线程可以执行了。
2：isAlive
/**
* Tests if this thread is alive. A thread is alive if it has
* been started and has not yet died.
*
* @return true if this thread is alive;
* false otherwise.
*/
public final native boolean isAlive();
判断当前的线程是否处于活动状态，活动状态就是线程已经启动且尚未终止。线程处于正在运行或准备开始运行的状态，就认为线程是“存活”的。
public class Mythread7_2 extends Thread {
public Mythread7_2() {
System.out.println("Mythread7_2---begin");
System.out.println("Thread.currentThread().getName()=" + Thread.currentThread().getName());
System.out.println("Thread.currentThread().isAlive()=" + Thread.currentThread().isAlive());
System.out.println("this.getName()=" + this.getName());
System.out.println("this.isAlive()=" + this.isAlive());
System.out.println("Mythread7_2---end");
}
@Override
public void run() {
System.out.println("run---begin");
System.out.println("Thread.currentThread().getName()=" + Thread.currentThread().getName());
System.out.println("Thread.currentThread().isAlive()=" + Thread.currentThread().isAlive());
System.out.println("this.getName()=" + this.getName());
System.out.println("this.isAlive()=" + this.isAlive());
System.out.println("run---end");
}
}
@Test
public void test7_2() {
Mythread7_2 c = new Mythread7_2();
System.out.println("main begin t1 isAlive=" + c.isAlive());
c.setName("A");
c.start();
System.out.println("main end t1 isAlive=" + c.isAlive());
}
执行结果：
Mythread7_2---begin Thread.currentThread().getName()=main Thread.currentThread().isAlive()=true this.getName()=Thread-0 this.isAlive()=false Mythread7_2---end main begin t1 isAlive=false run---begin Thread.currentThread().getName()=A Thread.currentThread().isAlive()=true this.getName()=A this.isAlive()=true run---end main end t1 isAlive=false
可以看出：只有在当前实例对象调用了start方法时，this.isAlive()才返回true。
3：interrupt
/**
* Interrupts this thread.
*
* Unless the current thread is interrupting itself, which is
* always permitted, the {@link #checkAccess() checkAccess} method
* of this thread is invoked, which may cause a {@link
* SecurityException} to be thrown.
*
*
If this thread is blocked in an invocation of the {@link
* Object#wait() wait()}, {@link Object#wait(long) wait(long)}, or {@link
* Object#wait(long, int) wait(long, int)} methods of the {@link Object}
* class, or of the {@link #join()}, {@link #join(long)}, {@link
* #join(long, int)}, {@link #sleep(long)}, or {@link #sleep(long, int)},
* methods of this class, then its interrupt status will be cleared and it
* will receive an {@link InterruptedException}.
*
*
If this thread is blocked in an I/O operation upon an {@link
* java.nio.channels.InterruptibleChannel InterruptibleChannel}
* then the channel will be closed, the thread's interrupt
* status will be set, and the thread will receive a {@link
* java.nio.channels.ClosedByInterruptException}.
*
*
If this thread is blocked in a {@link java.nio.channels.Selector}
* then the thread's interrupt status will be set and it will return
* immediately from the selection operation, possibly with a non-zero
* value, just as if the selector's {@link
* java.nio.channels.Selector#wakeup wakeup} method were invoked.
*
*
If none of the previous conditions hold then this thread's interrupt
* status will be set.
*
* Interrupting a thread that is not alive need not have any effect.
*
* @throws SecurityException
* if the current thread cannot modify this thread
*
* @revised 6.0
* @spec JSR-51
*/
public void interrupt() {
if (this != Thread.currentThread())
checkAccess();
synchronized (blockerLock) {
Interruptible b = blocker;
if (b != null) {
interrupt0(); // Just to set the interrupt flag
b.interrupt(this);
return;
}
}
interrupt0();
}
interrupt并不会立刻停止线程，interrupt0的作用仅仅是为当前线程打一个中断的标志。
4：isInterrupted
/**
* Tests whether this thread has been interrupted. The interrupted
* status of the thread is unaffected by this method.
*
* A thread interruption ignored because a thread was not alive
* at the time of the interrupt will be reflected by this method
* returning false.
*
* @return true if this thread has been interrupted;
* false otherwise.
* @see #interrupted()
* @revised 6.0
*/
public boolean isInterrupted() {
return isInterrupted(false);
}
测试线程是否已经中断，但不清除状态标识。这个和interrupted()方法区别就是不清除状态标识。
参考文献
1：《Java并发编程的艺术》
2：《Java多线程编程核心技术》