线程基础知识01-Thread类,Runnable接口

常见面试题：创建一个线程的常用方法有哪些？Thread创建线程和Runnable创建线程有什么区别？

答案通常集中在，继承类和实现接口的差别上面；

如果深入问一些问题：1.要执行的任务写在run()方法中，为什么要用start()方法启动？等等问题

简单的问题还是可以回答一哈子，但是涉及到深入些的问题，就只能看看源码，才能更好的回答问题了：

1.为啥线程要用start()方法启动？

首先要从Thread类的源码入手：

• Thread类实现了Runnable接口

public
class Thread implements Runnable {

• 可以看出，Thread类里面有一个Runnable接口的元素，通过构造方法给Thread类里面的Runnable元素赋值；

   private Runnable target;
   public Thread(Runnable target) {
      init(null, target, "Thread-" + nextThreadNum(), 0);
  }
  private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
       //省略部分代码
        this.group = g;
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();
      //......省略部分代码
        this.target = target;//注意这里的赋值操作
        setPriority(priority);
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

• run方法很简单，就是判断是不是通过构造函数传入了Runnable接口,可以看出，线程的方法并没有执行；

 @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }

• start（）方法中通过调用start0（）执行Run()方法；类似于this.run();报错后也是忽略状态

 public synchronized void start() {
        if (threadStatus != 0)
            throw new IllegalThreadStateException();

        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 */
            }
        }
    }

    private native void start0();

• 可以借鉴的地方：
  
  将任务封装在单独的接口中，可以多种实现，通过统一的方法去调用；

interface A {
  run(){)
}
class B implements A {
 private A a;
 B(){}
 B(A a){this.a = a}
 run(){
   if(a != null ){a.run}
 }
 start(){this.run()}
}

2.Thread的几种状态

• 类中给出的几种状态

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;
    }

下面这个图能更好的理解线程的状态

（图片来自《java并发编程艺术》）



1.线程开启，存在两种情况，运行状态和临时阻塞状态（调用方法，但没有获取到锁的情况下）；

运行状态，具有执行资格和执行权；临时阻塞状态，只有执行资格没有执行权；

2.线程运行完之后有两种状态：冻结/等待和TERMINATED，

冻结状态释放执行权和执行资格。调用wait()或sleep()方法

3.interceptor和stop之间的区别

stop:终结一个线程不会保证资源释放，比较暴力；

interceptor:线程被中断了，可以通过isInterceptor方法获取是否被终端；但是抛异常，或者isItercepter方法后，终端状态会被清楚；

比较好的线程终止方案，可以增加一个标识符eg:flag=true的判断，要中断的时候，将标识符设置位false即可；

4.什么是自旋锁？什么是无锁编程？

CAS简单了解： https://www.cnblogs.com/perferect/p/12761558.html

5.死锁

死锁的定义：一组互相竞争资源的线程因互相等待，导致“永久”阻塞的现象。

最常出现死锁的情况是，当两个或多个锁互相嵌套，一个锁内部进行其他锁的竞争；

class test{
  private static final Object obj1 = new Object();
  private static final Object obj2 = new Obbject();
  public static void main(String[] args){
      new Thread(){
        public void run(){
          synchronized(obj1){
               Thread.sleep(100);
               synchronized(obj2){
                   Thread.sleep(200);
                 }
           }
        }
      }.start();
      new Thread(){
        public void run(){
          synchronized(obj2){
               Thread.sleep(100);
               synchronized(obj1){
                   Thread.sleep(200);
                 }
           }
        }
      }.start();      

  }
}