public class ReentrantLock implements Lock, java.io.Serializable {
//ReentrantLock 有两种锁：公平锁，非公平锁
private final Sync sync;
//并发包基本 都是基于aqs
abstract static class Sync extends AbstractQueuedSynchronizer {...}
//非公平锁
static final class NonfairSync extends Sync {...}
//公平锁
static final class FairSync extends Sync {...}
//默认非公平锁
public ReentrantLock() {
sync = new NonfairSync();
}
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
}
先看看lock方法（非公平为例）：
public void lock() {
sync.lock();
}
final void lock() {
//这边首先要知道 state 是个锁定标志，0 说明是空闲
//如果空闲，修改为 1，设置当前线程获取锁
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
//获取锁
acquire(1);
}
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
从字面理解：尝试获取锁，如果失败，则加入获取锁的队列，加入之前 需要先创建node节点 ，默认是独占式的，这边先声明aqs有两种锁模式（共享式，独占式），这里可以看到ReentrantLock是独占式的；
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
//再次尝试获取锁，如果失败说明出现并发
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
//考虑到ReentrantLock可以重入锁 ，获取锁跟释放锁都是成双成对出现，
//对上线做一个校验，如果重入锁 返回true
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
如果获取 锁失败，回到acquire()方法，加入 获取锁队列，先看增加节点的方法：
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
Node pred = tail;
if (pred != null) {
//如果尾部node不为空，则把新增的node加到尾部，添加也是基于CAS
//如果添加失败，说明出现并发，走enq
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
private Node enq(final Node node) {
//如果是FIFO,是从head的下个node开始 ！！
for (;;) {
//这里是死循环，确保把新增的节点加到tail
Node t = tail;
if (t == null) {
//如果尾部为空，new一个node为头部，尾部也为这个头部的节点
if (compareAndSetHead(new Node()))
tail = head;
} else {
//把新增node加到尾部
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
节点创建完，然后是加到 队列
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)) {
//如果上一个节点刚好是头节点，也许已经释放锁，尝试获取锁
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
//检查前一个节点的状态，看当前获取锁失败的线程是否需要挂起。
parkAndCheckInterrupt())
//如果需要，借助JUC包下的LockSopport类的静态方法Park挂起当前线程。
//直到被唤醒。
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
return true;
if (ws > 0) {
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
public static void park(Object blocker) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, 0L);//0:永久
setBlocker(t, null);
}
上面有提到Node，其实它是 aqs很重要的内部 结构
abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private transient volatile Node head;
private transient volatile Node tail;
private volatile int state;
static final class Node {
static final Node SHARED = new Node();
static final Node EXCLUSIVE = null;
static final int CANCELLED = 1;//节点取消
static final int SIGNAL = -1;//节点等待触发
static final int CONDITION = -2;//节点等待条件
static final int PROPAGATE = -3;//节点状态需要向后传播。
//有上面四种状态 只有当前节点的前一个节点为SIGNAL时，才能当前节点才能被挂起。
volatile int waitStatus;
volatile Node prev;
volatile Node next;
volatile Thread thread;
Node nextWaiter;
}
