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应用样例
ThreadA、ThreadB、ThreadC 拜访如下逻辑
ReentrantLock lock = new ReentrantLock();
// == 1. 加锁
lock.lock();
... 省略业务解决...
// == 2. 开释
lock.unlock();非偏心加锁过程
偏心形式，无 ThreadD 局部逻辑，会间接入队
后续都在具体解释这张图
一、非偏心加锁
1. 状态批改
// ## 状态：拜访线程会采纳 cas 的形式批改 state 的值，加锁过程 0 ->1
private volatile int state;
// ## 持有线程：state 批改胜利的线程，将被记录。比方，exclusiveOwnerThread=ThreadA
private transient Thread exclusiveOwnerThread;
# NonfairSync 非偏心实现
final void lock() {
// == 1.cas 批改 state 状态 0->1（插队 1）if (compareAndSetState(0, 1))
// state 批改胜利批改持有线程 exclusiveOwnerThread = ThreadA
setExclusiveOwnerThread(Thread.currentThread());
else
// == 2. 构建队列，并阻塞线程
acquire(1);
}2. 队列构建
### public final void acquire(int arg) {
// a- 尝试获取，尝试批改 state 状态（未获取胜利持续后续逻辑）if (!tryAcquire(arg)
// b2- 排队获取
&& acquireQueued(
// b1- 新增期待节点，构建“独占”模式队列
addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();}a- 尝试获取（可能插队的地位）
java.util.concurrent.locks.ReentrantLock.NonfairSync#tryAcquire
java.util.concurrent.locks.ReentrantLock.Sync#nonfairTryAcquire
final boolean nonfairTryAcquire(int acquires) {final Thread current = Thread.currentThread();
int c = getState();
// ## 插队地位
if (c == 0) {if (compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current);
return true;
}
}
// ## 重入，state++
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;
}b1- 新增期待节点，构建“独占”模式队列
class Node {
/** 独占 */
static final Node EXCLUSIVE = null;
// 指向线程
volatile Thread thread;
volatile Node prev;
volatile Node next;
static final int SIGNAL = -1;
java.util.concurrent.locks.AbstractQueuedSynchronizer#addWaiter
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;
// == 2. 队列不为空，节点尾插
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
// == 1. 队列初始构建
enq(node);
return node; // 返回尾节点
}
//== 1. 队列初始构建
java.util.concurrent.locks.AbstractQueuedSynchronizer#enq
private Node enq(final Node node) {for (;;) {
Node t = tail;
// -- A、初始化构建，头尾指针指向空 Node
if (t == null) {if (compareAndSetHead(new Node()))
tail = head;
}
// -- B、尾插
else {
node.prev = t;
// cas 批改尾节点指向
if (compareAndSetTail(t, node)) {
t.next = node;
return t; // 返回头节点
}
}
}
}b2- 排队获取
java.util.concurrent.locks.AbstractQueuedSynchronizer#acquireQueued
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
// 循环中
for (;;) {final Node p = node.predecessor();
// ### 前置节点是头节点，有机会尝试获取
//（联合下一个 if 判断，会自旋两次，也就是说有两次尝试获取机会）if (p == head && tryAcquire(arg)) {setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
// ### 第 1 次将 waitstatus 设置成 signal 返回 false
// ### 第 2 次判断 waitstatus==signal 返回 true
if (shouldParkAfterFailedAcquire(p, node)
// === 线程阻塞（将来唤醒时，从此处继续执行）&& parkAndCheckInterrupt())
interrupted = true;
}
} finally {if (failed)
cancelAcquire(node);
}
}
###
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
// -- 第二次调用
if (ws == Node.SIGNAL)
return true;
if (ws > 0) {
do {node.prev = pred = pred.prev;} while (pred.waitStatus > 0);
pred.next = node;
}
// -- 第一次调用
else {compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
===
private final boolean parkAndCheckInterrupt() {LockSupport.park(this);
// 以后线程是否被中断
return Thread.interrupted();}二、开释
java.util.concurrent.locks.ReentrantLock#unlock
java.util.concurrent.locks.AbstractQueuedSynchronizer#release
{
// == 1.state 还原，exclusiveOwnerThread 清空
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
// == 2.“解除阻塞”执行胜利的节点
unparkSuccessor(h);
return true;
}
return false;
}
1.state 还原，exclusiveOwnerThread 清空
protected final boolean tryRelease(int releases) {
// 加锁时线程重入，state++。因而解锁时，state--
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
// state 归 0 时，开释线程援用
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}2.“解除阻塞”执行胜利的节点
private void unparkSuccessor(Node node) {
int ws = node.waitStatus;
// 加锁时，ws=SIGNAL，也就是 -1。当初改成 0
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;
}
// ## 开释 s 节点，也就是 head 的下一个节点
if (s != null)
LockSupport.unpark(s.thread);
}三、偏心加锁
差异一
# FairSync 偏心实现
final void lock() {
// 无插队操作，间接构建队列
acquire(1);
}再比照下刚刚的非偏心实现，只有 else 局部
# NonfairSync 非偏心实现
final void lock() {
// == 1.cas 批改 state 状态 0->1（插队 1）if (compareAndSetState(0, 1))
// state 批改胜利批改持有线程 exclusiveOwnerThread = ThreadA
setExclusiveOwnerThread(Thread.currentThread());
### 偏心实现只有这部分逻辑
else
// == 2. 构建队列，并阻塞线程
acquire(1);
}差异二
public final void acquire(int arg) {if (!tryAcquire(arg)
&& acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();}
protected final boolean tryAcquire(int acquires) {final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {// ### 偏心实现（多了!hasQueuedPredecessors()）：要求无排队状况才有资格尝试进行后续 cas 操作
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;
}
}
正文完
