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;
}
}