ReentrantLock实现原理
在多线程操作过程中,锁是保证原子性和可见性的重要方式,synchronized关键字是隐式获取锁的方式,Lock类比synchronized关键字更加灵活,是显示获取锁的方式。本文将详细了解ReentrantLock类。
1. ReentrantLock基本使用
使用该类比较简单,初始化该类后,使用其提供的API去获取锁和释放锁,如下:
public class ReentrantLockDemo {
private static Integer count = 0;
private static Lock lock = new ReentrantLock();
public static void increment() {
lock.lock();
count++;
lock.unlock();
}
public static void main(String[] args) {
for (int i = 0; i < 1000; i++) {
new Thread(ReentrantLockDemo::increment).start();
}
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(count);
}
}
使用上述lock和unlock方法,能保证同时只有一个线程能获取锁,其他线程处于阻塞队列中,当释放锁后,其他线程能再次获取锁,以此来保证锁区域的原子性,也能保证前一个线程的操作对后续线程操作的可见性。
2. ReentrantLock底层实现
先来看一下源码中该类的结构图:
ReentrantLock继承了AQS(AbstractQueuedSynchronizer)类,并且定义了同步器对象Sync。AQS类是实现锁的基础对象,在该类中定义了线程的同步状态及一个FIFO的阻塞队列。
首先,来看ReentrantLock构造方法:
public ReentrantLock() {
sync = new NonfairSync();
}
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
通过上述构造方法构建使用的同步器,默认是采用非公平锁的方式,可以通过构造传参进行选择。公平锁严格遵守FIFO模型,最先进入阻塞队列的线程在锁释放后最先执行。而非公平锁在锁释放后,任意线程均可抢占锁。
lock()方法源码如下:
public void lock() {
sync.lock();
}
先来分析非公平锁的方式:
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);
}
...
}
在NonfairSync.lock()方法中,会调用Unsafe
类的CAS方法修改线程状态,若能修改成功,则表示当前线程获得锁,会记录下当前线程信息;若CAS调用返回false,则调用AQS里面的acquire()方法
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
这里的tryAcquire()会调用ReentrantLock#NonfairSync中的tryAcquire方法,源码如下:
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
abstract static class Sync extends AbstractQueuedSynchronizer {
...
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
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;
}
...
}
在这个过程中,会再次判断线程的状态,若线程是无锁状态,则再次通过CAS获取锁。若当前线程再次获取锁,则直接增加获取锁的次数,这里是实现可重入锁的核心方法。
若还是获取不到锁,回到AQS中的acquire方法,会将当前线程组装一个Node节点,然后添加到阻塞队列中
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
private Node addWaiter(Node mode) {
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;
}
}
enq(node);
return node;
}
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
...
}
通过AQS类中定义的compareAndSetHead和compareAndSetTail方法,将获取锁失败的线程构建成Node,并添加至一个双向链表中,然后在添加至阻塞队列时,将当前线程阻塞
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
...
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
这里会借助LockSupport.park阻塞当前线程。当锁释放之后才会唤醒处于阻塞的线程。
public void unlock() {
sync.release(1);
}
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
AQS类中的tryRelease()方法也是一个抽象方法,具体实现由子类进行实现,源码如下:
abstract static class Sync extends AbstractQueuedSynchronizer {
...
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
// 删除独占锁标识
setExclusiveOwnerThread(null);
}
// 将state状态复原
setState(c);
return free;
}
}
state记录了获取锁的次数,如果是重入锁,需要等当前线程将锁完全释放后,才会将独占锁的线程标识清空。
在AQS的release方法中,若当前线程释放锁,且阻塞队列中有节点时,则会唤醒处于阻塞状态的下一个线程
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
private void unparkSuccessor(Node node) {
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
// 获取下一个节点,若下一个节点为空或者为取消状态,则从尾部往前找到当前节点后第一个未取消的节点
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;
}
// 释放锁
if (s != null)
LockSupport.unpark(s.thread);
}
这里借助LockSupport.unpark唤醒处于双向链表中的下一个节点,直到所有的锁释放,程序执行结束。
而公平锁在获取锁时会检查阻塞队列中是否有值,若有值,则会将当前线程加入阻塞队列:
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;
}
其他的操作与非公平锁一致。