AQS结构剖析
- 双向链表 + waitStatus的int值
锁的结构：
- 实现Lock接口
- 组合AQS进行并发状态控制
为什么使用双向链表实现？
因为链表移除和添加比较方便，只需要改动prev和next节点的指向即可，移除和添加都只需要操作一次，时间复杂度为O(1)。如果使用数组去实现，随着数据量的增加每次操作需要移动的次数也会更重
waitStatus的int值是什么？有什么用？
waitStatus
volatile int waitStatus AQS核心实现，等待状态，它有几种状态值：CANCELLED、SIGNAL、CONDITION、PROPAGATE
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;
// AQS核心实现，等待状态
volatile int waitStatus;
volatile Node prev;
volatile Node next;
volatile Thread thread;
Node nextWaiter;
}
**CANCELLED：**由锁状态变成取消状态，这个时候就可以被gc回收了
SIGNAL: 插入名为4的节点到3和2之间，然后将4节点的前继节点也就是2的waitStatus改成SIGNAL状态
其余的节点相信大家直接看释义就能明白了
源码分析
参考流程图，我们按照程序流程来分析源代码
ReentrantLock
ReentrantLock支持公平锁和非公平锁，我们可以通过它的构造函数来控制选择哪种锁。默认无参构造是非公平锁实现
ReentrantLock提供的公平锁FairSync和非公平锁NonfairSync都是继承自AbstractQueuedSynchronizer就是AQS
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
/** Synchronizer providing all implementation mechanics */
private final Sync sync;
/**
* 默认无参构造非公平锁实现
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
public ReentrantLock() {
sync = new NonfairSync();
}
/**
* 通过boolean fair控制选择公平锁和非公平锁
* Creates an instance of {@code ReentrantLock} with the
* given fairness policy.
*
* @param fair {@code true} if this lock should use a fair ordering policy
*/
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
/**
* Base of synchronization control for this lock. Subclassed
* into fair and nonfair versions below. Uses AQS state to
* represent the number of holds on the lock.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* lock加锁方法，从这里作为入口开始分析
* Performs {@link Lock#lock}. The main reason for subclassing
* is to allow fast path for nonfair version.
*/
abstract void lock();
......
lock
abstract void lock();是模板方法模式，由子类实现，在ReentrantLock中它的实现有公平锁和非公平锁两者，这里我们只有关注非公平锁的实现
先来比较一下公平锁和非公锁的区别在哪？
重点看lock()方法
非公平锁NonfairSync.lock()它一上来就先通过if (compareAndSetState(0, 1))cas去抢锁
如果抢锁成功：
则把当前线程，也就是自身，通过setExclusiveOwnerThread设置为当前独占锁的线程（占用锁的线程）
如果抢锁失败：
则走acquire(1)方法，继续抢锁，在失败就通过enq加入阻塞队列队尾
公平锁FairSync.lock()，则是直接调用acquire(1)方法，内部实现大致是先通过状态判断有无线程正在占用，如果没有也就是state == 0则继续通过hasQueuedPredecessor判断当前线程前面有没有其它等待的线程，如果没有在去抢锁，如果有则返回false，通过enq加入阻塞队列队尾
独占锁：同一时刻只有一个线程可以持有锁，其它线程未获取到锁时，会被阻塞
/**
* Sync object for non-fair locks
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
/**
* Sync object for fair locks
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
acquire(1);
}
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}
tryAcquire
我们继续分析非公平锁的实现
程序调用了acquire(1)之后，会先通过tryAcquire(1)去尝试获取锁，重点看一下它的实现
它是先通过int c = getState()获取锁标记，判断是否有锁
如果锁状态等于0，那说明无锁
则去通过cas抢锁，抢锁成功，则把自己设置为独占锁的线程
如果锁状态不等于0，说明有锁
先走else if的判断当前线程和独占锁的线程是否为同一线程，如果是，则直接拿到锁，也就是重入锁的特性，ReentrantLock就是重入独占锁，拿到锁之后继续给state累加1，表示有锁
如果else if也判断失败，则返回false，tryAcquire尝试获取锁失败，这时走acquireQueued(addWaiter(Node.EXCLUSIVE), arg)
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
// State: 锁标记 0是无锁、大于等于1是有锁状态()
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
加入队列队尾
在addWaiter中，能看到它是先创建了一个当前线程的node节点，然后获取到了tail节点，也就是尾节点，如果tail节点存在，那么则将当前线程创建的node节点的prev，也就是当前线程的前置节点指向现有的tail尾节点
然后通过cas抢锁，抢锁成功
把自己设置为尾节点，在把之前的尾节点的next指向现在的node节点，并返回node节点出去
抢锁失败
则通过enq方法，自旋加入队列。简单的说enq之前的代码是一种快速尝试插入节点，加入队列队尾的方法
那么为什么需要enq自旋入队列呢？
因为在这里是存在锁竞争的，所以需要抢锁，在操作
/**
* 当前线程入队列，并返回当前线程对应的node节点
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
// 以给定模式构造节点。mode有两种：EXCLUSVIE（独占）和SHARED（共享）
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
// 上面执行失败，执行这里自旋加入队列，队尾
enq(node);
return node;
}
/**
* Inserts node into queue, initializing if necessary. See picture above.
* @param node the node to insert
* @return node's predecessor
*/
private Node enq(final Node node) {
// CAS"自旋"，直到成功加入队尾
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
// 队列为空，创建一个空的标志节点作为head节点，并将tail也指向它
// 创建第一个节点，头尾都是自己
if (compareAndSetHead(new Node()))
tail = head;
} else { // 正常流程，加入队尾
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
判断前驱节点释放为head
/**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
*
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true; // 标记是否成功拿到资源
try {
boolean interrupted = false; // 标记等待过程中是否被中断过
// 自旋
for (;;) {
// 当前节点的前驱节点
final Node p = node.predecessor();
// 如果前驱节点是head，尝试获取资源（可能是head释放完资源唤醒当前线程），当然也可能被interrupt)
if (p == head && tryAcquire(arg)) {
// 竞争锁成功
// 设置当前线程为head节点
setHead(node);
// 出队
p.next = null; // help GC
failed = false; // 成功获取资源
return interrupted; // 返回等待过程中是否被中断过
}
// park，挂起线程
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
接下来我们看shouldParkAfterFailedAcquire方法
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
// pred是前置节点，Node是当前节点
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
// 获取前置节点的waitStatus
int ws = pred.waitStatus;
// SIGNAL的释义，请看上面的waitStatus状态值图示
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) { // 取消调度，cancel了
/*
*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
// 看下面的图示
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0); // 循环执行，直到waitStatus不大于0
// 前置的next == 当前节点
pred.next = node;
} else {
/*
* 如果前驱正常，那就把前驱的状态设置为SIGNAL
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
if (ws > 0) 图示
线程挂起阻塞
挂起线程
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
// 只有Unpark时才能解锁
LockSupport.park(this);
return Thread.interrupted();
}
unlock 唤醒后继节点
那么lock获取锁的流程已经完事了，现在就是解锁的过程了
我们看看unLock();
public void unlock() {
sync.release(1);
}
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
// 接触被park的线程
unparkSuccessor(h);
return true;
}
return false;
}
protected final boolean tryRelease(int releases) {
// 减去自旋增加的状态值
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) { // 释放
free = true;
setExclusiveOwnerThread(null);
}
// 状态最终需要设置回0
setState(c);
return free;
}
真正的解锁，解除被挂起的线程，唤醒后继节点unparkSuccessor
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}