Java并发

基础问题

线程和进程的含义及区别

• 线程
• 进程
  

线程的状态和变迁

如何创建一个线程？

继承Thread类，实现Runable接口,Callable和Future

start() 和 run() 区别

如下图所示，start()方法只能调用一次，重复调用会抛出IllegalThreadStateException,run()方法可以重复调用

为什么调用start()会执行run()方法

见下面JDK Thread start()中的注释；总结就是，start()方法会让JVM调用该Thread的run()方法，并在新线程上执行，即主线程调用start()过后返回并继续执行，新的线程将会被创建并用来执行run()方法

start()方法中call了一个native的start0()

native方法是指该方法的实现由非Java语言实现，如C或C++，不提供实现体，是个原生态方法

Java语言本身不能对操作系统底层进行访问和操作，但可以通过JNI(Java Native Interface)接口调用其他语言来实现对底层的访问，JNI是JDK的一部分

为什么不能直接调用run()方法

不会创建新的线程，该run()方法就是个普通的方法将在主线程执行，一般来说直接调用run()是一个bug或者失误

/**
     * 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.
     *
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
     * @see        #run()
     * @see        #stop()
     */
    public synchronized void start() {
        /**
         * This method is not invoked for the main method thread or "system"
         * group threads created/set up by the VM. Any new functionality added
         * to this method in the future may have to also be added to the VM.
         *
         * A zero status value corresponds to state "NEW".
         */
        if (threadStatus != 0)
            throw new IllegalThreadStateException();

        /* Notify the group that this thread is about to be started
         * so that it can be added to the group's list of threads
         * and the group's unstarted count can be decremented. */
        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();

    /**
     * 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.
     *
     * @see     #start()
     * @see     #stop()
     * @see     #Thread(ThreadGroup, Runnable, String)
     */
    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }

yield()方法有什么作用？

yield()方法只是让线程从RUNNING状态变为READY状态，但是也可能变为READY后又立刻被CPU调度器执行(变为RUNNING)；如下图JDK注释说的，yield()只是告诉cpu调度器，我可以让出我的时间片，但是cpu调度器可以忽视这个消息;一般来说这个方法没有很多应该使用的场景，多是用于测试或者debug。

sleep()和yield()方法有什么区别？

sleep()让线程从RUNABLE状态变为TIMED_WAITING状态，让出CPU的时间片，不考虑线程优先级，yield()只会让优先级高的运行；sleep的线程不会丧失任何monitor的所有权

为什么sleep()和yield()方法是静态的？

sleep()和yield()都是谁调谁sleep/yield也都是自愿的，如果是实例方法会造成很多混乱，即我可以让你这个线程sleep

/**
     * A hint to the scheduler that the current thread is willing to yield
     * its current use of a processor. The scheduler is free to ignore this
     * hint.
     *
     * <p> Yield is a heuristic attempt to improve relative progression
     * between threads that would otherwise over-utilise a CPU. Its use
     * should be combined with detailed profiling and benchmarking to
     * ensure that it actually has the desired effect.
     *
     * <p> It is rarely appropriate to use this method. It may be useful
     * for debugging or testing purposes, where it may help to reproduce
     * bugs due to race conditions. It may also be useful when designing
     * concurrency control constructs such as the ones in the
     * {@link java.util.concurrent.locks} package.
     */
    public static native void yield();

    /**
     * Causes the currently executing thread to sleep (temporarily cease
     * execution) for the specified number of milliseconds, subject to
     * the precision and accuracy of system timers and schedulers. The thread
     * does not lose ownership of any monitors.
     *
     * @param  millis
     *         the length of time to sleep in milliseconds
     *
     * @throws  IllegalArgumentException
     *          if the value of {@code millis} is negative
     *
     * @throws  InterruptedException
     *          if any thread has interrupted the current thread. The
     *          <i>interrupted status</i> of the current thread is
     *          cleared when this exception is thrown.
     */
    public static native void sleep(long millis) throws InterruptedException;

sleep()和wait()方法有什么区别？

wait()是Object的实例方法，有几个重点：

• 当前线程必须持有该object的锁(monitor)

• wait()方法使得当前线程阻塞，并释放该object的锁(monitor)

• 当前线程将等待其他线程调用该object的notify()或者notifyAll()的方法重新获得锁并继续执行下去

• 响应中断

• wait()方法必须用在循环里面

  为什么wait()方法必须用在循环里面，即用while(condition not hold)而不是if(condition not hold):

  当有多个线程由于相同condition not hold在wait()这行代码处阻塞等待持锁的时候，一旦condition change某个线程拿到锁了从wait()往下执行，又改变了condition,释放锁，由于另外个阻塞的线程是if判断，不再判断condition，拿到锁再处理时会发生错误。假设condition是array size > 0 而wait()之后会进行remove，那么就会出现IndexOutOfArray的问题

显而易见的相同点为：都使得当前线程阻塞了（TIMED_WAITING/WAITING状态）且都响应中断；不同点为：一个是Thread的静态方法，一个是Object的实例方法；wait()需要当前线程持有该object的monitor,sleep是当前线程自愿的行为；wait()调用后当前线程会释放monitor,sleep不会释放任何monitor；sleep自动唤醒，wait需要其他线程调用该object的notify和notifyAll()方法

/**
     * Causes the current thread to wait until another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object.
     * In other words, this method behaves exactly as if it simply
     * performs the call {@code wait(0)}.
     * <p>
     * The current thread must own this object's monitor. The thread
     * releases ownership of this monitor and waits until another thread
     * notifies threads waiting on this object's monitor to wake up
     * either through a call to the {@code notify} method or the
     * {@code notifyAll} method. The thread then waits until it can
     * re-obtain ownership of the monitor and resumes execution.
     * <p>
     * As in the one argument version, interrupts and spurious wakeups are
     * possible, and this method should always be used in a loop:
     * <pre>
     *     synchronized (obj) {
     *         while (&lt;condition does not hold&gt;)
     *             obj.wait();
     *         ... // Perform action appropriate to condition
     *     }
     * </pre>
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the {@code notify} method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of the object's monitor.
     * @throws  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The <i>interrupted
     *             status</i> of the current thread is cleared when
     *             this exception is thrown.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#notifyAll()
     */
    public final void wait() throws InterruptedException {
        wait(0);
    }

join()方法怎么使用？

1. join()方法不是一个静态方法，是Thread类的实例方法
2. 当前线程会等待t.join()的t线程执行完成后才继续执行
3. 响应中断

/**
     * Waits for this thread to die.
     *
     * <p> An invocation of this method behaves in exactly the same
     * way as the invocation
     *
     * <blockquote>
     * {@linkplain #join(long) join}{@code (0)}
     * </blockquote>
     *
     * @throws  InterruptedException
     *          if any thread has interrupted the current thread. The
     *          <i>interrupted status</i> of the current thread is
     *          cleared when this exception is thrown.
     */
    public final void join() throws InterruptedException {
        join(0);
    }

守护线程和用户线程有什么区别？

1. 主线程结束后用户线程还会继续运行，JVM存活
2. 如果没有用户线程，都是守护线程，那么JVM结束（所有的线程都会结束）

守护线程是一种特殊的线程，在后台默默地完成一些系统性的服务，比如垃圾回收线程、JIT线程都是守护线程。与之对应的是用户线程，用户线程可以理解为是系统的工作线程，它会完成这个程序需要完成的业务操作。如果用户线程全部结束了，意味着程序需要完成的业务操作已经结束了，系统可以退出了。所以当系统只剩下守护进程的时候，java虚拟机会自动退出

什么是线程安全，有哪些线程安全程度？

点我

操作系统可以保证每个进程只能访问分配给自己的地址空间。而每个进程中都有一块内存空间（堆内存）是公共的，是所有线程都能访问的，就像小区的公园一样。操作系统也会为每个线程分配自己的内存空间（栈内存），只有自己这个线程能访问。

线程的安全其实是内存的安全。

如何保证线程安全？

1. 放到栈内存，例如局部变量。但是也就只有自己能访问了，当变量从（方法内的）局部变量变为（类的）成员变量的时候，也就是从栈内存到公共的堆内存，就可能会出现问题
2. 人人有份。ThreadLocal,每个Thread都有自己的一个map存储变量。这些变量虽然整体是放到堆内存的，但是由于自己拷贝一份到自己的map,自己处理自己的，就像是本地的一样，也就安全了。
3. 只能看不能摸。例如，常量或者只读变量。
4. 制定规则，先入为主。例如可重入锁。
5. 地广人稀，CAS。

什么是中断

另外一个问题，我们应该

如何安全地结束一个线程？

stop等方法已经被舍弃了

1. 使用退出标志，用一个while()循环，再设置一个boolean标志
2. 使用中断thread.interrupt()将设置该线程中断位为true，包含两种，一种本质就是1，用while循环加上isInterrupted()来判断；一种通过catch InterruptedException来处理

interrupt()方法只是改变中断状态，不会中断一个正在运行的线程。需要用户自己去监视线程的状态为并做处理。支持线程中断的方法（也就是线程中断后会抛出interruptedException的方法）就是在监视线程的中断状态，一旦线程的中断状态被置为“中断状态”，就会抛出中断异常。这一方法实际完成的是，给受阻塞的线程发出一个中断信号，这样受阻线程检查到中断标识，就得以退出阻塞的状态。

更确切的说，如果线程被Object.wait, Thread.join和Thread.sleep三种方法之一阻塞，此时调用该线程的interrupt()方法，那么该线程将抛出一个 InterruptedException中断异常（该线程必须事先预备好处理此异常），从而提早地终结被阻塞状态。如果线程没有被阻塞，这时调用 interrupt()将不起作用，直到执行到wait(),sleep(),join()时,才马上会抛出 InterruptedException

/**
     * Interrupts this thread.
     *
     * <p> Unless the current thread is interrupting itself, which is
     * always permitted, the {@link #checkAccess() checkAccess} method
     * of this thread is invoked, which may cause a {@link
     * SecurityException} to be thrown.
     *
     * <p> If this thread is blocked in an invocation of the {@link
     * Object#wait() wait()}, {@link Object#wait(long) wait(long)}, or {@link
     * Object#wait(long, int) wait(long, int)} methods of the {@link Object}
     * class, or of the {@link #join()}, {@link #join(long)}, {@link
     * #join(long, int)}, {@link #sleep(long)}, or {@link #sleep(long, int)},
     * methods of this class, then its interrupt status will be cleared and it
     * will receive an {@link InterruptedException}.
     *
     * <p> If this thread is blocked in an I/O operation upon an {@link
     * java.nio.channels.InterruptibleChannel InterruptibleChannel}
     * then the channel will be closed, the thread's interrupt
     * status will be set, and the thread will receive a {@link
     * java.nio.channels.ClosedByInterruptException}.
     *
     * <p> If this thread is blocked in a {@link java.nio.channels.Selector}
     * then the thread's interrupt status will be set and it will return
     * immediately from the selection operation, possibly with a non-zero
     * value, just as if the selector's {@link
     * java.nio.channels.Selector#wakeup wakeup} method were invoked.
     *
     * <p> If none of the previous conditions hold then this thread's interrupt
     * status will be set. </p>
     *
     * <p> Interrupting a thread that is not alive need not have any effect.
     *
     * @throws  SecurityException
     *          if the current thread cannot modify this thread
     *
     * @revised 6.0
     * @spec JSR-51
     */
    public void interrupt() {
        if (this != Thread.currentThread())
            checkAccess();

        synchronized (blockerLock) {
            Interruptible b = blocker;
            if (b != null) {
                interrupt0();           // Just to set the interrupt flag
                b.interrupt(this);
                return;
            }
        }
        interrupt0();
    }

/**
     * Tests whether the current thread has been interrupted.  The
     * <i>interrupted status</i> of the thread is cleared by this method.  In
     * other words, if this method were to be called twice in succession, the
     * second call would return false (unless the current thread were
     * interrupted again, after the first call had cleared its interrupted
     * status and before the second call had examined it).
     *
     * <p>A thread interruption ignored because a thread was not alive
     * at the time of the interrupt will be reflected by this method
     * returning false.
     *
     * @return  <code>true</code> if the current thread has been interrupted;
     *          <code>false</code> otherwise.
     * @see #isInterrupted()
     * @revised 6.0
     */
    public static boolean interrupted() {
        return currentThread().isInterrupted(true);
    }

    /**
     * Tests whether this thread has been interrupted.  The <i>interrupted
     * status</i> of the thread is unaffected by this method.
     *
     * <p>A thread interruption ignored because a thread was not alive
     * at the time of the interrupt will be reflected by this method
     * returning false.
     *
     * @return  <code>true</code> if this thread has been interrupted;
     *          <code>false</code> otherwise.
     * @see     #interrupted()
     * @revised 6.0
     */
    public boolean isInterrupted() {
        return isInterrupted(false);
    }

    /**
     * Tests if some Thread has been interrupted.  The interrupted state
     * is reset or not based on the value of ClearInterrupted that is
     * passed.
     */
    private native boolean isInterrupted(boolean ClearInterrupted);

interrupted和isInterrupted方法的区别？

如上图所示，interrupted()是线程的静态方法，返回当前线程的中断标记且会重置中断标记；isInterrupted()是实例方法，返回this的中断标记且不会重置中断标记。

可重入锁 & synchronized

可重入锁和synchronized的主要区别

1. 机制是不一样的：synchronized是java内置的关键字，在JVM层面实现，系统会监控锁的释放，且自动释放，同步执行完或发生异常将释放锁；lock是JDK代码实现的，需要手动在finally模块释放，并可以非阻塞地获取锁。
2. 性能不一样：竞争激烈的情况下lock会比synchronized好；竞争不激烈的情况下synchronized性能好，且synchronized会从偏向锁-->轻量级锁-->重量级锁升级
3. 场景范围不一样：使用lock，线程2等待线程1释放锁的时候，可以不用一直等待，synchronized做不到
4. 总的来说，lock提供了比synchronized更多的功能：如下图PPT所示

• synchronized 同步格式

synchronized(需要一个任意的对象(锁)){
     代码块中放操作共享数据的代码；
} 

• 关于Lock

1. Lock其实只是一个接口

2. ReentrantLock是唯一实现了Lock接口的类

可重入锁的实现？

先看JUC中的[AQS]

什么是synchronized的锁升级？

出自这里

Java对象在内存中的存储结构包括：请求头，实例数据，填充数据；对象头包含Mark Word(hashCode, GC分代年龄，锁信息)，Class Metadata address(指向对象类型数据的指针)，Array Length(该对象为数组时，数组的长度)

• 偏向锁

  当对象被创建出来的时候，就有了偏向锁的标志位“01”，但状态为0，即被创建的对象的偏向锁不生效；但是，当线程执行到临界区(critical section)的时候，此时会利用CAS操作将线程ID插入到Markword中，并修改偏向锁的标志位，即状态为0，且能知道是哪个线程拥有了偏向锁。

  当此线程执行之后，若这个同步块代码又被进入了，则会：

  1. 判断当前线程是否与Markword当中的线程id一致，若一致则继续执行
  2. 若不一致，检查对象是否还是可偏向的，即偏向锁的状态
  3. 如果未偏向，则利用CAS竞争锁，也就是第一次获取锁时的操作
  4. 如果已经偏向了，可能需要重新偏向，或者大部分时候升级为轻量级锁

  偏向锁存在的意义：大部分时候都是同一个线程进入同一个同步块，那么这个时候就不需要再有加锁解锁的开销；偏向锁实际上是偏向第一个拿到锁的线程。

• 升级为轻量级锁

  • 锁撤销-有开销

    1. 在一个安全的点停止拥有锁的线程
    2. 遍历线程栈，存在锁记录的话，需要修复锁记录和Markword使其变成无锁状态
    3. 唤醒当前线程，升级成轻量级锁

  • 轻量级锁：锁标志位为“00”,此时Markword bitfield被替换为指向LockRecord的指针，之前的bitfield将会复制到创建的这个LockRecord里面，LockRecord有个owner的指针指向对象

  • 轻量级锁又分为自旋锁和自适应自旋锁

  • 自旋锁：

    当有线程竞争锁时，会在原地循环等待，直到锁被释放可以立刻获取锁，而这个原地循环会消耗CPU。适用于同步代码块执行很快的场景。经验表明，大部分同步代码块都执行很快。设置一个原地循环的最大限制次数，如果超过就升级为重量级锁。默认为10次，可以通过-XX:PreBlockSpin来进行更改。

  • 自适应自旋锁：

    即可动态调整自旋等待的次数而不是个固定值。

  • 轻量级锁也被称为非阻塞同步，乐观锁

• 升级为重量级锁

  • 重量级锁以来对象内部monitor，monitor又依赖于操作系统的mutex锁，此时的标志位为“10”,bitfield将会指向mutex的指针
  • 为什么重量级锁开销大？
    1. 此时，等待的线程会被阻塞，不会消耗CPU但是阻塞或者唤醒一个线程时都需要操作系统来帮忙，这就需要从用户态转换到内核态，这个转换很耗时。
  • 重量级锁又被称为阻塞同步，悲观锁

为什么竞争激烈的情况下lock比synchronized性能好？

volatile关键字

volatile是什么意思？原理是什么？

CAS的特点是什么？

监视器(Monitor)和Condition区别

操作系统在面对 进程/线程 间的同步的时候所支持的同步原语中，信号量(semaphore)和互斥量(mutex)是最重要的。在使用mutex进行并发控制时，要非常小心地控制其down和up的操作，否则将引起死锁。在此基础出现了更高层次的同步原语，使我们不需要亲自去操作变量进行阻塞和唤醒，这个更高级的同步原语就是monitor---也是编程语言在语法上提供的语法糖，如何实现属于编译器的工作，不是操作系统的范畴。

• monitor的基本元素

  • 临界区：互斥进入临界区
  • monitor对象和锁：monitor object有相应数据结构保存被阻塞的线程，和基于mutex的锁
  • 条件变量和定义在monitor对象上的wait和notify操作

• java中的每一个对象都有一个监视器（Monitor）

  • 使用synchronized关键字来圈定临界区，实现互斥的界限

  • monitor object:

    synchronized需要指定一个对象与之关联，如果synchronized修饰的是实例方法，那么其实关联的就是this，如果修饰的是类方法，关联的对象就是this.class，这个关联的对象就是monitor object。

    锁：

    Java对象存储在内存中，分别分为三个部分：对象头，实例数据和对齐填充，在对象头中就保存了锁标识。

  • wait和notify的操作：

    这些方法的实现是JVM内部基于C++实现的一套机制，原理如下图：总结就是，waitThread(即调用object.wait()方法的线程)先要获取object的锁(也就是为什么需要配合synchronized关键字使用)，如果获取成功，执行到object.wait()这行代码处，该线程就放进了这个object的waitQueue里面，表示在等待中。如果获取失败，该线程将被放进synchronizedQueue中，想要继续尝试获取锁。

    当调用object.notifyAll()的方法时，所有等在waitQueue里的线程将移动到synchronizedQueue里面去和那些waitThread没有获取锁的线程一起尝试获取锁。

    一旦某个线程获取成功了，就继续往下执行。

• 总结：Monitor是一套机制，它界于操作系统和我们实际编程中间，提供并发编程的方式。

J.U.C.

AQS

AQS是什么？

AQS叫做队列同步器(AbstractQueuedSynchronizer)，是用来构建锁和同步组件的基础框架。

• AQS有一个内部类Node，每个Node主要就是存一个Thread，Thread的状态，父节点prev和子节点next

static final class Node {
    /** Marker to indicate a node is waiting in shared mode */
    static final Node SHARED = new Node();
    /** Marker to indicate a node is waiting in exclusive mode */
    static final Node EXCLUSIVE = null;

    /** waitStatus value to indicate thread has cancelled */
    static final int CANCELLED =  1;
    /** waitStatus value to indicate successor's thread needs unparking */
    static final int SIGNAL    = -1;
    /** waitStatus value to indicate thread is waiting on condition */
    static final int CONDITION = -2;
    /**
     * waitStatus value to indicate the next acquireShared should
     * unconditionally propagate
     */
    static final int PROPAGATE = -3;

    /**
     * Status field, taking on only the values:
     *   SIGNAL:     The successor of this node is (or will soon be)
     *               blocked (via park), so the current node must
     *               unpark its successor when it releases or
     *               cancels. To avoid races, acquire methods must
     *               first indicate they need a signal,
     *               then retry the atomic acquire, and then,
     *               on failure, block.
     *   CANCELLED:  This node is cancelled due to timeout or interrupt.
     *               Nodes never leave this state. In particular,
     *               a thread with cancelled node never again blocks.
     *   CONDITION:  This node is currently on a condition queue.
     *               It will not be used as a sync queue node
     *               until transferred, at which time the status
     *               will be set to 0. (Use of this value here has
     *               nothing to do with the other uses of the
     *               field, but simplifies mechanics.)
     *   PROPAGATE:  A releaseShared should be propagated to other
     *               nodes. This is set (for head node only) in
     *               doReleaseShared to ensure propagation
     *               continues, even if other operations have
     *               since intervened.
     *   0:          None of the above
     *
     * The values are arranged numerically to simplify use.
     * Non-negative values mean that a node doesn't need to
     * signal. So, most code doesn't need to check for particular
     * values, just for sign.
     *
     * The field is initialized to 0 for normal sync nodes, and
     * CONDITION for condition nodes.  It is modified using CAS
     * (or when possible, unconditional volatile writes).
     */
    volatile int waitStatus;

    /**
     * Link to predecessor node that current node/thread relies on
     * for checking waitStatus. Assigned during enqueuing, and nulled
     * out (for sake of GC) only upon dequeuing.  Also, upon
     * cancellation of a predecessor, we short-circuit while
     * finding a non-cancelled one, which will always exist
     * because the head node is never cancelled: A node becomes
     * head only as a result of successful acquire. A
     * cancelled thread never succeeds in acquiring, and a thread only
     * cancels itself, not any other node.
     */
    volatile Node prev;

    /**
     * Link to the successor node that the current node/thread
     * unparks upon release. Assigned during enqueuing, adjusted
     * when bypassing cancelled predecessors, and nulled out (for
     * sake of GC) when dequeued.  The enq operation does not
     * assign next field of a predecessor until after attachment,
     * so seeing a null next field does not necessarily mean that
     * node is at end of queue. However, if a next field appears
     * to be null, we can scan prev's from the tail to
     * double-check.  The next field of cancelled nodes is set to
     * point to the node itself instead of null, to make life
     * easier for isOnSyncQueue.
     */
    volatile Node next;

    /**
     * The thread that enqueued this node.  Initialized on
     * construction and nulled out after use.
     */
    volatile Thread thread;

    /**
     * Link to next node waiting on condition, or the special
     * value SHARED.  Because condition queues are accessed only
     * when holding in exclusive mode, we just need a simple
     * linked queue to hold nodes while they are waiting on
     * conditions. They are then transferred to the queue to
     * re-acquire. And because conditions can only be exclusive,
     * we save a field by using special value to indicate shared
     * mode.
     */
    Node nextWaiter;

    /**
     * Returns true if node is waiting in shared mode.
     */
    final boolean isShared() {
        return nextWaiter == SHARED;
    }

    /**
     * Returns previous node, or throws NullPointerException if null.
     * Use when predecessor cannot be null.  The null check could
     * be elided, but is present to help the VM.
     *
     * @return the predecessor of this node
     */
    final Node predecessor() throws NullPointerException {
        Node p = prev;
        if (p == null)
            throw new NullPointerException();
        else
            return p;
    }

    Node() {    // Used to establish initial head or SHARED marker
    }

    Node(Thread thread, Node mode) {     // Used by addWaiter
        this.nextWaiter = mode;
        this.thread = thread;
    }

    Node(Thread thread, int waitStatus) { // Used by Condition
        this.waitStatus = waitStatus;
        this.thread = thread;
    }
}

• AQS本身维护一个Node节点的双向链表队列，和一个表征同步状态的state字段

/**
     * Head of the wait queue, lazily initialized.  Except for
     * initialization, it is modified only via method setHead.  Note:
     * If head exists, its waitStatus is guaranteed not to be
     * CANCELLED.
     */
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
    private transient volatile Node tail;

    /**
     * The synchronization state.
     */
    private volatile int state;

AQS独占式和共享式的含义？

• 独占式：同一时刻只有一个线程持有同步状态，如ReentrantLock的实现

  独占式获取同步状态过程：

• 共享式：共享资源可以被多个线程同时占有，如ReadWriteLock，CountdownLatch

AQS的模板模式是什么？

• AQS只是一个基础框架，tryAcquire(int arg)， tryRelease(int arg)，tryAcquireShared(int arg)，tryReleaseShared(int arg)，isHeldExclusively()都没有具体实现，因为公平锁有公平锁的实现方式，非公平锁有非公平锁的实现方式等，需要子类去实现

todo

简述并发工具Semaphore, CountdownLatch, CyclicBarrier的特点使用场景

线程安全的集合有哪些？

ConcurrentHashMap

BlockingQueue

CopyOnWriteArrayList

ConcurrentLinkedQueue

线程池