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一文看尽Java-Thread

一、前言

主要分成两部说起:Thread源码解读和常见面试题解答，废话不多说开始；

二、源码解读

首先看下构造函数，构造函数都是通过调用init方法对属性进行初始化，主要是对线程组、线程名字、栈大小等信息进行初始化；init内部通过调用currentThread本地方法，获取当前的线程，这个本地方法封装在JVM中,有兴趣的可以看下这个这个链接查找下JVM实现 https://hg.openjdk.java.net/jdk8u ，接下来对ThreadGroup的判断，如果没有传入线程组的话，第一是使用SecurityManager中的ThreadGroup，如果从SecurityManager 中获取不到ThreadGroup()，那么就从当前线程中获取线程组，最后做了检验和些参数的赋值，整体上相对比较简单；


 
  private void init(ThreadGroup g, Runnable target, String name,
                    long stackSize) {
      init(g, target, name, stackSize, null);
  }
  private void init(ThreadGroup g, Runnable target, String name,
                    long stackSize, AccessControlContext acc) {
      if (name == null) {
          throw new NullPointerException("name cannot be null");
      }
      this.name = name.toCharArray();
      Thread parent = currentThread();
      SecurityManager security = System.getSecurityManager();
      if (g == null) {
          /* Determine if it's an applet or not */
          /* If there is a security manager, ask the security manager
             what to do. */
          if (security != null) {
              g = security.getThreadGroup();
          }
          /* If the security doesn't have a strong opinion of the matter
             use the parent thread group. */
          if (g == null) {
              g = parent.getThreadGroup();
          }
      }
      /* checkAccess regardless of whether or not threadgroup is
         explicitly passed in. */
      g.checkAccess();
      /*
       * Do we have the required permissions?
       */
      if (security != null) {
          if (isCCLOverridden(getClass())) {
              security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
          }
      }
      g.addUnstarted();
      this.group = g;
      this.daemon = parent.isDaemon();
      this.priority = parent.getPriority();
      if (security == null || isCCLOverridden(parent.getClass()))
          this.contextClassLoader = parent.getContextClassLoader();
      else
          this.contextClassLoader = parent.contextClassLoader;
      this.inheritedAccessControlContext =
              acc != null ? acc : AccessController.getContext();
      this.target = target;
      setPriority(priority);
      if (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();
  }
  public Thread() {
      init(null, null, "Thread-" + nextThreadNum(), 0);
  }
  
  public Thread(Runnable target) {
      init(null, target, "Thread-" + nextThreadNum(), 0);
  }
  Thread(Runnable target, AccessControlContext acc) {
      init(null, target, "Thread-" + nextThreadNum(), 0, acc);
  }
 // 线程名
  public Thread(String name) {
      init(null, null, name, 0);
  }
  //线程组和线程名
  public Thread(ThreadGroup group, String name) {
      init(group, null, name, 0);
  }
  //线程任务，线程名
  public Thread(Runnable target, String name){
      init(null, target, name, 0);
  }
  // 线程组， 线程任务， 线程名 ，栈大小
  public Thread(ThreadGroup group, Runnable target, String name, long stackSize) {
      init(group, target, name, stackSize);
  }

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接下来看下主要的属性：


// 类加载的时候，调用本地的注册本地方静态方法， 这个方法是本地方法
  private static native void registerNatives();
  static {
      registerNatives();
  }
  private volatile char  name[];
  private int            priority;
  private Thread         threadQ;
  private long           eetop;
  /* Whether or not to single_step this thread. */
  private boolean     single_step;
  /* Whether or not the thread is a daemon thread. */
  // 设设置这个线程是否是守护线程
  private boolean     daemon = false;
  /* JVM state */
  private boolean     stillborn = false;
  /* What will be run. */
  // 要执行的run方法的对象
  private Runnable target;
  /* The group of this thread */
  // 这个线程的线程组
  private ThreadGroup group;
  /* The context ClassLoader for this thread */
  // 这个线程的上下文类加载器
  private ClassLoader contextClassLoader;
  /* The inherited AccessControlContext of this thread */
  private AccessControlContext inheritedAccessControlContext;
  /* For autonumbering anonymous threads. */
  private static int threadInitNumber;
  private static synchronized int nextThreadNum() {
      return threadInitNumber++;
  }
  /* ThreadLocal values pertaining to this thread. This map is maintained
   * by the ThreadLocal class. */
  ThreadLocal.ThreadLocalMap threadLocals = null;
  /*
   * InheritableThreadLocal values pertaining to this thread. This map is
   * maintained by the InheritableThreadLocal class.
   */
  ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
  /*
   * The requested stack size for this thread, or 0 if the creator did
   * not specify a stack size.  It is up to the VM to do whatever it
   * likes with this number; some VMs will ignore it.
   */
   // 给这个线程设置的栈的大小,默认为0 
  private long stackSize;
  /*
   * JVM-private state that persists after native thread termination.
   */
  private long nativeParkEventPointer;
  /*
   * Thread ID
   */
   
   //线程id
  private long tid;
  /* For generating thread ID */
  private static long threadSeqNumber;
  /* Java thread status for tools,
   * initialized to indicate thread 'not yet started'
   */
  private volatile int threadStatus = 0;
  private static synchronized long nextThreadID() {
      return ++threadSeqNumber;
  }
  /**
   * The argument supplied to the current call to
   * java.util.concurrent.locks.LockSupport.park.
   * Set by (private) java.util.concurrent.locks.LockSupport.setBlocker
   * Accessed using java.util.concurrent.locks.LockSupport.getBlocker
   */
  volatile Object parkBlocker;
  /* The object in which this thread is blocked in an interruptible I/O
   * operation, if any.  The blocker's interrupt method should be invoked
   * after setting this thread's interrupt status.
   */
  private volatile Interruptible blocker;
  private final Object blockerLock = new Object();
  /* Set the blocker field; invoked via sun.misc.SharedSecrets from java.nio code
   */
  void blockedOn(Interruptible b) {
      synchronized (blockerLock) {
          blocker = b;
      }
  }
  /**
   * The minimum priority that a thread can have.
   */
   // 线程执行的最低优先级 为1
  public final static int MIN_PRIORITY = 1;
 /**
   * The default priority that is assigned to a thread.
   */
   // 线程默认的执行优先级为 5
  public final static int NORM_PRIORITY = 5;
  /**
   * The maximum priority that a thread can have.
   */
   // 线程执行的最高的优先级为 10
  public final static int MAX_PRIORITY = 10;

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最后介绍下方法的作用和线程状态，源码都比较简单，没必进行过多的介绍，都是通过调用JVM的本地方法实现；

线程状态：

一文看尽Java-Thread

三、常见面试题

1.线程与进程的区别?

进程是资源分配最小的单位，线程是CPU调度最小的单位；

线程属于进程，共享进程分配的资源；

进程属于抢占式调度，资源不相互共享；

2.start和run的区别?

run是Thread的一个普通的方法；

start方法会创建一个新的子线程并启动；

3.sleep与wait的区别?

sleep是Thread方法，wait是Object的方法；

wait方法只能在synchroized方法或者块中使用；

Thread.sleep只会让出CPU，不会改变锁的行为；

Object.wait不仅会让出CPU，同时还会释放占有同步资源的锁；

4.线程状态的转化?

图中将 WAITING 和 TIMED_WAITING 两个状态合并为 WAITING ，没有分开，大家不要搞错；

一文看尽Java-Thread

5.如何处理线程的返回值?

主线程等待法，使用while等待主线程返回值；

join阻塞当前线程以等待子线程；

通过FuTureTask获取子线程的返回值；


public class MyCallable implements Callable<String> {
    @Override
    public String call() throws Exception {

        String value="test";
        System.out.println("start");
        Thread.sleep(5000);
        System.out.println("end");
        return value;

    }
}
public class FutureTaskDemo {

    public static void main(String[] main) throws ExecutionException, InterruptedException {
        FutureTask<String> futureTask=new FutureTask<String>(new MyCallable());

        new Thread(futureTask).start();

        if (!futureTask.isDone()){
            System.out.println("waiting");
        }
        System.out.println("return"+futureTask.get());
    }
}

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通过线程池获取返回值；


public class ThreadPoolDemo {
    public static void main(String[] args){
        ExecutorService executorService= Executors.newCachedThreadPool();
        Future<String> futureTask=executorService.submit(new MyCallable());
        if (!futureTask.isDone()){
            System.out.println("wait");
        }
        try {
            System.out.println(futureTask.get());
        }catch (InterruptedException ex){
            ex.printStackTrace();
        }catch (ExecutionException ex){
            ex.printStackTrace();
        }finally {
            executorService.shutdown();
        }

    }
}

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6.Thread和Runnable？

Thread是类，Runnable是接口，Thread是Runnable实现；

类的继承单一原则，Runnable是更高层次的抽象；