进程与线程
线程是进程的一部分，线程本身不会单独存在。进程是操作系统资源分配的基本单位，线程则是资源分配的最小单位。
线程存在的意义在于提升CPU的运行效率。一个CPU单位时间只能执行一个任务，假如这个任务执行过程中阻塞，那CPU资源就被闲置。因此加入线程，在JVM层面每个线程拥有自己的虚拟机栈与程序计数器。多核CPU下，CPU执行中线程可以被挂起，根据线程的程序计数器又能从上次执行到的地方继续执行。
线程状态
操作系统进程五态
新建状态NEW:进程被创建时所处的状态。
就绪状态READY:进程已经具备了可执行条件，等待CPU调度。
运行状态Running:进程占用CPU正在执行。
阻塞状态Blocked:进程受到通信或同步操作影响，不得不放弃CPU使用。
终止状态Terminated:进程运行结束或出现严重错误。收回占用资源。
就绪-》运行：CPU分配给某个就绪的进程。
运行-》阻塞：正在运行中的进程需要等待某些事件的发生。
运行-》就绪：CPU分配的时间耗尽或其他优先级更高的进程插队
java线程状态
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;
}
NEW：还未调用start()的线程处于NEW状态。
RUNABLE: 包含了处于就绪Ready和运行Running
WAITING、TIMED_WAITING只是加入了超时时间。
线程创建
有三种创建线程的方式：实现Runable接口、继承Thread类、实现Callable
callable可以拿到线程执行的结果
public class Call implements Callable {
@Override
public Object call() throws Exception {
return "test";
}
public static void main(String[] args) throws ExecutionException, InterruptedException {
FutureTask futureTask = new FutureTask(new Call());
new Thread(futureTask).start();
System.out.println(futureTask.get());
}
}
线程通知机制
Object类是所有类的父类，它拥有wait()、notify()等方法。
要先使用这些方法，首先必须要拿到该对象的监视器锁（在synchronized修饰的代码块中）。
wait():线程挂起，并释放对象的监视器锁。加入锁对象的等待队列。
notify():随机唤醒一个该对象等待队列中的线程，使之进入同步队列去争抢监视器锁。notify并不会释放锁
public static void main(String[] args) throws InterruptedException {
Object o = new Object();
Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0;i<50 ;i++){
synchronized (o){
System.out.println(Thread.currentThread().getName()+"线程一"+i);
o.notify();
try {
o.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}});
Thread threadB = new Thread(new Runnable() {
@Override
public void run() {
for(int j = 0; j<50 ;j++){
synchronized (o){
System.out.println(Thread.currentThread().getName()+"线程二"+j);
o.notify();
try {
o.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}});
threadA.start();
Thread.sleep(100);
threadB.start();
}
threadA先启动，之后主线程谁100ms，确保线程A先拿到对象o的监视器锁。线程A输出后，调用notify()唤醒处于等待队列的线程，此时并没又唤醒任何线程，然后wait()阻塞。此后，主线程睡眠结束，线程B开始执行。线程B输出后，执行notify()将线程A从等待队列唤醒进入同步队列，然后线程Bwait()释放锁，让线程A执行。
threadA先启动，之后主线程谁100ms，确保线程A先拿到对象o的监视器锁。线程A输出后，调用notify()唤醒处于等待队列的线程，此时并没又唤醒任何线程，然后wait()阻塞。此后，主线程睡眠结束，线程B开始执行。线程B输出后，执行notify()将线程A从等待队列唤醒进入同步队列，然后线程Bwait()释放锁，让线程A执行。
sleep()释放同步锁。
转载：https://blog.csdn.net/qq_40365331/article/details/106180547