Java Thread你应该知道的一切
线程的知识点真的很多，想要全面掌握，需要日积月累，今天就请大家跟随我，一起走进Thread的世界。
想要走进Thread,那么就从源码开始吧。
Thead State 线程的状态
这个知识点，是你必须知道的，线程那些状态呢？
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;
}
上面的枚举类是Thread内部枚举，里面包含了线程的几种状态，那么我们来看看，都是什么状态。
NEW
Thread state for a thread which has not yet started
public class ThreadTest {
public static void main(String[] args){
Thread thread = new Thread();
Thread.State state = thread.getState();
System.out.println(state.toString());
}
}
当运行以上代码的时候，打印结果是 NEW，这样我们可以理解，线程创建以后，在没有调用start方法以前，Thread的状态是NEW.
RUNNABLE
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.
public class ThreadTest {
public static void main(String[] args){
Thread thread = new Thread();
thread.start();
Thread.State state = thread.getState();
System.out.println(state.toString());
}
}
当运行以上代码的时候，打印的结果是RUNNABLE，当前线程从NEW状态转到了RUNNABLE状态。
在网上看到好多人在写Thread状态的时候，还有运行中状态，这是不正确的，如何用代码证明呢？
public class ThreadTest {
public static void main(String[] args){
Thread thread = new Thread(new MyThread());
thread.start();
Thread.State state = thread.getState();
System.out.println(state.toString());
}
static class MyThread implements Runnable{
public void run() {
int count = 0;
for (int i=0;i<10000;i++){
count = count+i;
System.out.println(Thread.currentThread().getState().toString());
}
}
}
}
运行以上代码，你会发现，线程根本没有所谓的运行中的状态，CPU已经计算结果了
BLOCKED
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}
阻塞状态,当前线程在等待一个monitor lock。
1.调用synchronized，使线程进入阻塞状态。
2.调用wait方法，使线程进入阻塞状态。
WAITING
使线程进入WAITING状态的几种场景：
1.调用了wait方法，没有设置过期时间
2.调用Thread.join方法，没有设置过期时间
3.调用了LockSupport#park
TIMED_WAITING
使线程进入TIMED_WAITING状态的几种场景：
1.调用了Thread.sleep方法
2.调用了Object.wati方法，设置了过期时间
3.调用了Thread.join方法，设置了过期时间
4.调用了LockSupport#parkNanos
5.调用了LockSupport#parkUntil
TERMINATED
线程已经执行完成，就进入了线程终止状态。
线程的优先级
当多个线程的优先级相同的时候，它们使抢夺CPU资源。如果，你想让某个线程有限执行，你就可以设置它的优先级，可以通过Thread.setPriority设置线程优先级，优先级一共有10个级别，线程默认的优先级使5
/**
* The minimum priority that a thread can have.
*/
public final static int MIN_PRIORITY = 1;
/**
* The default priority that is assigned to a thread.
*/
public final static int NORM_PRIORITY = 5;
/**
* The maximum priority that a thread can have.
*/
public final static int MAX_PRIORITY = 10;
public final void setPriority(int newPriority) {
ThreadGroup g;
checkAccess();
if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {
throw new IllegalArgumentException();
}
if((g = getThreadGroup()) != null) {
if (newPriority > g.getMaxPriority()) {
newPriority = g.getMaxPriority();
}
setPriority0(priority = newPriority);
}
}
守护线程
