Thread的run()和start()

首先看下面的代码演示

public static void main(String[] args) {
    Thread newThread = new Thread(){
        @Override
        public void run() {
            super.run();
            System.out.println(Thread.currentThread().getName());
        }
    };
    newThread.run();
    newThread.start();
}

打印结果为：
main
Thread-0

如代码所示，如果直接调用run()方法，代码是在Thread对象所在的当前线程执行的。而调用start()方法，run()方法里的代码，才是在新线程里执行的。关于这个区别，可以看看Thread类中关于这样个方法的注解

* Causes this thread to begin execution; the Java Virtual Machine
* calls the <code>run</code> method of this thread.
* <p>
* The result is that two threads are running concurrently: the
* current thread (which returns from the call to the
* <code>start</code> method) and the other thread (which executes its
* <code>run</code> method).
* <p>
* It is never legal to start a thread more than once.
* In particular, a thread may not be restarted once it has completed
* execution.

public synchronized void start() {....}

     * If this thread was constructed using a separate
     * <code>Runnable</code> run object, then that
     * <code>Runnable</code> object's <code>run</code> method is called;
     * otherwise, this method does nothing and returns.
     * <p>
     * Subclasses of <code>Thread</code> should override this method.
    
    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }

Thread的状态

提到这个是因为，网上的一些文章对Thread的状态，说法不一致，可能是每个人的理解不同，此处我直接查看了jdk中的源码

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;
    }

需要注意的是，有些文章会提到，Thread.start()调用后会先进入“就绪状态”，但从源码可以看出，Thread.start()的调用会让线程进入RUNNABLE状态。
另外，源码注释中提到对WAITING和TIMED_WAITING状态的区别。
WAITING状态的触发：

• 调用Object.wait()方法，且未指定timeout参数
• 调用Thread.join()方法，且未指定timeout参数
• 调用LockSupport.park方法

TIMED_WAITING状态的触发：

• 调用Thread.sleep(timeout)方法
• 调用Object.wait(timeout)方法
• 调用Thread.join(timeout)方法
• 调用LockSupport.parkNanos方法
• 调用LockSupport.parkUntil方法

ThreadLocal

在分析过Handler机制的同学，应该会在查看Looper源码时，看到这个类。

public final class Looper {
    ...
    // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
   ...

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 *
 * <p>For example, the class below generates unique identifiers local to each
 * thread.
 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
 * and remains unchanged on subsequent calls.
 * <pre>
 * import java.util.concurrent.atomic.AtomicInteger;
**/
public class ThreadLocal<T> {

从类名也可以看出，它是用来存储一个线程对象的本地化数据的。具体来说，就是在线程的整个执行过程中，都有可能用到的一些数据，通常也可以称之为上下文(Context)，它是一种状态，可以是用户信息，任务信息之类的。因为在线程任务的执行过程中可能会调用到很多方法，有些方法还是层层调用，如果这些方法中要用到这些数据，除了将数据作为参数传进去之外，从ThreadLocal中读取和保存数据，就是java提供的一种方式。
看看它在Looper中的使用

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

通过get()来读取数据，set()方法来保存数据。
实际上，可以把ThreadLocal看成一个全局的Map<Thread,Object>，每个线程获取ThreadLocal变量是，总是使用Thread对象本身作为key。

Object threadLocalValue = threadLocalMap.get(Thread.currentThread());

因此，ThreadLocal相当于给每个线程都开辟了一个独立的存储空间，各个线程的ThreadLocal关联的实例互不干扰。另外，相比于直接在Thread中直接定义成员变量，这样的机制，减少了数据实例和Thread对象的耦合性。
特别注意ThreadLocal一定要在线程任务执行完毕时清除，这是因为当前线程执行完相关代码后，很可能会被重新放入线程池，如果ThreadLocal没有被清楚，该线程执行其他代码时，会把之前执行任务的状态带进去。

Thread和Runnable

在java中，一般实现创建线程执行任务的方法有两种，一种是直接继承Thread类，另一种是实现Runnable接口。区别在于：

• java是单继承，但可以实现多个接口。所以从类的扩展来看，实现Runnable接口会比继承Thread类，更灵活。
• Runnable只是一个接口，它本身并不具备创建线程执行任务的功能。它只是定义了一个可执行的任务。实际上，任务的执行需要Thread调用Runnable定义的run方法。
  上面THread类中默认实现的run()方法，显示了两者的关系，其中的target就是Runnable对象

/* What will be run. */
    private Runnable target;

Runnable和Callable

Callable和Runnable都是用来定义可异步执行的任务的接口，Runnable是JDK1.0就有的API，Callable是JDK1.5增加的API。两者的区别在于：

• Callable的call()方法有返回值并且可以抛出异常，而Runnable的run()方法不具备这些特征。
• 执行Callable任务，可以拿到一个Future对象，表示异步执行的结果。通过Future对象可以了解任务执行情况，可取消任务的执行，也可以获得执行的结果。
• Thread类只支持执行Runnable接口，不支持执行Callable接口

FutureTask

FutureTask类实现了RunnableFuture<T>接口，而RunnbleFuture<T>接口继承了Runnable接口和Future<T>接口，也就是说FutureTask<T>类是同时实现了Runnable接口和Future<T>接口。所以它既能当做Runnable被线程执行，又能作为Future<T>得到Callable<T>的返回值。FutureTask还可以让调用者知道任务什么时候执行完，并获得线程执行完成后返回的结果。

public class FutureTask<V> implements RunnableFuture<V>{
  public FutureTask(Callable<V> callable) {
  }
  public FutureTask(Runnable runnable, V result) {
  }
}

public interface RunnableFuture<V> extends Runnable, Future<V> {
    void run();
}

下面是一个结合Callable、Future和FutureTask的常见用法

executorService = new ScheduledThreadPoolExecutor(5);
        Callable<String> task = new Callable<String>() {
            @Override
            public String call() throws Exception {
                Thread.sleep(2000);
                System.out.println(Thread.currentThread().getName());
                System.out.println("callable is running");
                return "callable is done";
            }
        };
        /**
         * 用法一
         */
//        Future future = executorService.submit(task);
//        executorService.shutdown();
//        future.get();
        /**
         * 用法二
         */
        FutureTask futureTask = new FutureTask(task);
        executorService.submit(futureTask);
        executorService.shutdown();
        /**
         * 用法三
         */
//        new Thread(futureTask).start();

        System.out.println(Thread.currentThread().getName());
        System.out.println("main is running");
        try {
            System.out.println(futureTask.get());
            System.out.println("main get result");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

synchronized的理解

我们通常用到synchronized关键字是这样

public void testSynchronized(){
        synchronized(this){
            System.out.println("test is start");
            try {
                Thread.sleep(2000);
                System.out.println("test is over");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

或者是这样

public synchronized void testSynchronized(){
        System.out.println("test is start");
        try {
            Thread.sleep(2000);
            System.out.println("test is over");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

也就是说，synchronized既可以加在一段代码上，也可以加在方法上。看上去像是第一种锁的是一个对象，而第二种锁的是方法里的代码。但实质上并不是这样。

synchronized锁住的是括号里的对象，而不是代码。对于非static的synchronized方法，锁的就是对象本身也就是this。synchronized(this)以及非static的synchronized方法，只能防止多个线程执行同一个对象的同步代码段。
当synchronized锁住一个对象后，别的线程如果也想拿到这个对象的锁，就必须等待这个线程执行完成释放锁，才能再次给对象加锁，这样才达到线程同步的目的。即使两个不同的代码段，都要锁同一个对象，那么这两个代码段也不能在多线程环境下同时运行。

再看下下面的代码

public void testSynchronized(){
        synchronized(Account.class){
            System.out.println("test is start");
            try {
                Thread.sleep(2000);
                System.out.println("test is over");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
......
......
for (int i = 0;i < 3;i++){
            Account account = new Account();
            Thread thread = new Thread(){
                @Override
                public void run() {
                    super.run();
                    account.testSynchronized();
                }
            };
            thread.start();
        }

代码中，每个子线程中，执行的都是不同的Account对象的testSynchronized方法。那么要让testSynchronized方法不被多线程同时执行，有两种方式可以实现：

• 如代码所示，用synchronized(Account.class)，实现了全局锁的效果，锁住了代码段。
• 也可以定义为静态方法，静态方法可以直接用类名加方法名调用，方法内无法使用this。所以锁的不是this对象，而是类的Class对象。static synchronized修饰的方法也相当于全局锁，锁住了方法内的代码。

本文参考
https://www.cnblogs.com/marsitman/p/11228684.html
https://www.liaoxuefeng.com/wiki/1252599548343744/1306581251653666