本文基于jdk1.8进行分析。
reentrantlock是一个可重入锁，在concurrenthashmap中使用了reentrantlock。
首先看一下源码中对reentrantlock的介绍。如下图。reentrantlock是一个可重入的排他锁，它和synchronized的方法和代码有着相同的行为和语义，但有更多的功能。reentrantlock是被最后一个成功lock锁并且还没有unlock的线程拥有着。如果锁没有被别的线程拥有，那么一个线程调用lock方法，就会成功获取锁并返回。如果当前线程已经拥有该锁，那么lock方法会立刻返回。这个可以通过isheldbycurrentthread方法和getholdcount方法进行验证。除了这部分介绍外，类前面的javadoc文档很长，就不在这里全部展开。随着后面介绍源码，会一一涉及到。
|
1
2
3
4
5
6
7
8
9
10
11
12
|
/**
* a reentrant mutual exclusion {@link lock} with the same basic
* behavior and semantics as the implicit monitor lock accessed using
* {@code synchronized} methods and statements, but with extended
* capabilities.
* <p>a {@code reentrantlock} is <em>owned</em> by the thread last
* successfully locking, but not yet unlocking it. a thread invoking
* {@code lock} will return, successfully acquiring the lock, when
* the lock is not owned by another thread. the method will return
* immediately if the current thread already owns the lock. this can
* be checked using methods {@link #isheldbycurrentthread}, and {@link
* #getholdcount}.
|
首先看一下成员变量，如下图。reentrantlock只有一个成员变量sync，即同步器，这个同步器提供所有的机制。sync是abstractqueuedsynchronizer的子类，同时，sync有2个子类，nonfairsync和fairsync，分别是非公平锁和公平锁。sync，nonfairesync和fairsync的具体实现后面再讲。
|
1
2
|
/** synchronizer providing all implementation mechanics **/
private final sync sync;
|
下面看一下构造函数。如下图。可以看到，reentrantlock默认是非公平锁，它可以通过参数，指定初始化为公平锁或非公平锁。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
/**
* creates an instance of {@code reentrantlock}.
* this is equivalent to using {@code reentrantlock(false)}.
**/
public reentrantlock() {
sync = new nonfairsync();
}
/**
* 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();
}
|
下面看一下reentrantlock的主要方法。首先是lock方法。如下图。lock方法的实现很简单，就是调用sync的lock方法。而sync的lock方法是个抽象的，具体实现在nonfairsync和fairsync中。这里我们先不展开讲，而是先读一下lock方法的注释，看看它的作用。lock方法的作用是获取该锁。分为3种情况。
1，如果锁没有被别的线程占有，那么当前线程就可以获取到锁并立刻返回，并把锁计数设置为1。
2，如果当前线程已经占有该锁了，那么就会把锁计数加1，立刻返回。
3，如果锁被另一个线程占有了，那么当前线程就无法再被线程调度，并且开始睡眠，直到获取到锁，在获取到到锁时，会把锁计数设置为1。
lockinterruptibly方法与lock功能类似，但lockinterruptibly方法在等待的过程中，可以响应中断。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
|
/**
* acquires the lock.
* <p>acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
* <p>if the current thread already holds the lock then the hold
* count is incremented by one and the method returns immediately.
* <p>if the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until the lock has been acquired,
* at which time the lock hold count is set to one.
**/
public void lock() {
sync.lock();
}
public void lockinterruptibly() throws interruptedexception {
sync.acquireinterruptibly(1);
}
|
下面，详细看一下非公平锁和公平锁中对lock函数的实现。如下图。下图同时列出了公平锁和非公平锁中lock的实现逻辑。从注释和代码逻辑中，都可以看出，非公平锁进行lock时，先尝试立刻闯入（抢占），如果成功，则获取到锁，如果失败，再执行通常的获取锁的行为，即acquire(1)。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
/**
* 非公平锁中的lock
* 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);
}
//公平锁中的lock
final void lock() {
acquire(1);
}
|
那么，我们首先了解下，非公平锁“尝试立刻闯入”，究竟做了什么。稍后再继续讲解通常的获取锁的行为。下图是立即闯入行为compareandsetstate(0, 1)的实现。从compareandsetstate函数的注释中，可以知道，如果同步状态值与期望值相等，那么就把它的值设置为updated值。否则同步状态值与期望值不相等，则返回false。这个操作和volatile有着相同的内存语义，也就是说，这个操作对其他线程是可见的。compareandsetstate函数注释里描述的功能，是通过unsafe.compareandswapint方法实现的，而unsafe.compareandswapint是一个native方法，是用c++实现的。那么继续追问，c++底层是怎么实现的？c++底层是通过cas指令来实现的。什么是cas指令呢？来自*的解释是，cas，比较和交换，compare and swap，是用用于实现多线程原子同步的指令。它将内存位置的内容和给定值比较，只有在相同的情况下，将该内存的值设置为新的给定值。这个操作是原子操作。那么继续追问，cas指令的原子性，是如何实现的呢？我们都知道指令时cpu来执行的，在多cpu系统中，内存是共享的，内存和多个cpu都挂在总线上，当一个cpu执行cas指令时，它会先将总线lock位点设置为高电平。如果别的cpu也要执行cas执行，它会发现总线lock位点已经是高电平了，则无法执行cas执行。cpu通过lock保证了指令的原子执行。
现在来看一下非公平锁的lock行为，compareandsetstate(0, 1)，它期望锁状态为0，即没有别的线程占用，并把新状态设置为1，即标记为占用状态。如果成功，则非公平锁成功抢到锁，之后setexclusiveownerthread，把自己设置为排他线程。非公平锁这小子太坏了。如果抢占失败，则执行与公平锁相同的操作。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
/**
* atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* this operation has memory semantics of a {@code volatile} read
* and write.
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. false return indicates that the actual
* value was not equal to the expected value.
**/
protected final boolean compareandsetstate(int expect, int update) {
// see below for intrinsics setup to support this
return unsafe.compareandswapint(this, stateoffset, expect, update);
}
public final native boolean compareandswapint(object var1, long var2, int var4, int var5);
|
下面看一下公平锁获取锁时的行为。如下图。这部分的逻辑有些多，请阅读代码中的注释进行理解。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
|
/**
* 公平锁的lock
**/
final void lock() {
acquire(1);
}
/**
* acquires in exclusive mode, ignoring interrupts. implemented
* by invoking at least once {@link #tryacquire},
* returning on success. otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryacquire} until success. this method can be used
* to implement method {@link lock#lock}.
* @param arg the acquire argument. this value is conveyed to
* {@link #tryacquire} but is otherwise uninterpreted and
* can represent anything you like.
**/
public final void acquire(int arg) {
/**
* acquire首先进行tryacquire()操作。如果tryacquire()成功时则获取到锁，即刻返回。
* 如果tryacquire()false时，会执行acquirequeued(addwaiter(node.exclusive), arg)
* 操作。如果acquirequeued(addwaiter(node.exclusive), arg)true时，则当前线程中断自己。
* 如果acquirequeued(addwaiter(node.exclusive), arg)false，则返回。
* 其中tryacquire()操作在nonfairsync中和fairsync中实现又有所区别。
**/
if (!tryacquire(arg) &&
acquirequeued(addwaiter(node.exclusive), arg))
selfinterrupt();
}
/**
* nonfairsync中的tryacquire。
* @param acquires
* @return
**/
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();
//首先获取当前同步状态值
int c = getstate();
if (c == 0) {
//c为0，表示目前没有线程占用锁。没有线程占用锁时，当前线程尝试抢锁，如果抢锁成功，则返回true。
if (compareandsetstate(0, acquires)) {
setexclusiveownerthread(current);
return true;
}
}
else if (current == getexclusiveownerthread()) {
//c不等于0时表示锁被线程占用。如果是当前线程占用了，则将锁计数加上acquires，并返回true。
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new error("maximum lock count exceeded");
setstate(nextc);
return true;
}
//以上情况都不是时，返回false，表示非公平抢锁失败。
return false;
}
/**
* fair version of tryacquire. don't grant access unless
* recursive call or no waiters or is first.
* 这个是公平版本的tryacquire
**/
protected final boolean tryacquire(int acquires) {
final thread current = thread.currentthread();
int c = getstate();
if (c == 0) {
//c=0时表示锁未被占用。这里是先判断队列中前面是否有别的线程。没有别的线程时，才进行cas操作。
//公平锁之所以公平，正是因为这里。它发现锁未被占用时，首先判断等待队列中是否有别的线程已经在等待了。
//而非公平锁，发现锁未被占用时，根本不管队列中的排队情况，上来就抢。
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;
}
/**
* acquires in exclusive uninterruptible mode for thread already in
* queue. used by condition wait methods as well as acquire.
* 当抢锁失败时，先执行addwaiter(node.exclusive)，将当前线程加入等待队列，再执行该方法。
* 该方法的作用是中断当前线程，并进行检查，知道当前线程是队列中的第一个线程，并且抢锁成功时，
* 该方法返回。
* @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;
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);
}
}
|
接下来是trylock方法。代码如下。从注释中我们可以理解到，只有当调用trylock时锁没有被别的线程占用，trylock才会获取锁。如果锁没有被另一个线程占用，那么就获取锁，并立刻返回true，并把锁计数设置为1. 甚至在锁被设置为公平排序的情况下，若果锁可用，调用trylock会立刻获取锁，而不管有没有别的线程在等待锁了。从这里我们总结出，不管可重入锁是公平锁还是非公平锁，trylock方法只会是非公平的。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
|
/**
* acquires the lock only if it is not held by another thread at the time
* of invocation.
* <p>acquires the lock if it is not held by another thread and
* returns immediately with the value {@code true}, setting the
* lock hold count to one. even when this lock has been set to use a
* fair ordering policy, a call to {@code trylock()} <em>will</em>
* immediately acquire the lock if it is available, whether or not
* other threads are currently waiting for the lock.
* this "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. if you want to honor
* the fairness setting for this lock, then use
* {@link #trylock(long, timeunit) trylock(0, timeunit.seconds) }
* which is almost equivalent (it also detects interruption).
* <p>if the current thread already holds this lock then the hold
* count is incremented by one and the method returns {@code true}.
* <p>if the lock is held by another thread then this method will return
* immediately with the value {@code false}.
* @return {@code true} if the lock was free and was acquired by the
* current thread, or the lock was already held by the current
* thread; and {@code false} otherwise
**/
public boolean trylock() {
return sync.nonfairtryacquire(1);
}
public boolean trylock(long timeout, timeunit unit)
throws interruptedexception {
return sync.tryacquirenanos(1, unit.tonanos(timeout));
}
|
接下来是释放锁的方法unlock。代码如下。unlock方式的实现，是以参数1来调用sync.release方法。而release方法是如何实现的呢？release方法首先会调用tryrelease方法，如果tryrelease成功，则唤醒后继者线程。而tryrelease的实现过程十分清晰，首先获取锁状态，锁状态减去参数（放锁次数），得到新状态。然后判断持有锁的线程是否为当前线程，如果不是当前线程，则抛出illegalmonitorstateexception。然后判断，如果新状态为0，说明放锁成功，则把持有锁的线程设置为null，并返回true。如果新状态不为0，则返回false。从tryrelease的返回值来看，它返回的true或false，指的是否成功的释放了该锁。成功的释放该锁的意思是彻底释放锁，别的线程就可以获取锁了。这里要认识到，即便tryrelease返回false，它也只是说明了锁没有完全释放，本次调用的这个释放次数值，依然是释放成功的。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
|
/**
* attempts to release this lock.
* <p>if the current thread is the holder of this lock then the hold
* count is decremented. if the hold count is now zero then the lock
* is released. if the current thread is not the holder of this
* lock then {@link illegalmonitorstateexception} is thrown.
* @throws illegalmonitorstateexception if the current thread does not
* hold this lock
**/
public void unlock() {
sync.release(1);
}
/**
* releases in exclusive mode. implemented by unblocking one or
* more threads if {@link #tryrelease} returns true.
* this method can be used to implement method {@link lock#unlock}.
* @param arg the release argument. this value is conveyed to
* {@link #tryrelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryrelease}
**/
public final boolean release(int arg) {
if (tryrelease(arg)) {
node h = head;
if (h != null && h.waitstatus != 0)
unparksuccessor(h);
return true;
}
return false;
}
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;
setexclusiveownerthread(null);
}
setstate(c);
return free;
}
/**
* 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);
}
|
接下来是newcondition方法。关于condition这里不展开介绍，只是了解下该方法的作用。如下图。该方法返回一个和这个锁实例一起使用的condition实例。返回的condition实例支持和object的监控方法例如wait-notify和notifyall相同的用法。
- 1，如果没有获取锁，调用condition的await，signal，signalall方法的任何一个时，会抛出illegalmonitorstateexception异常。
- 2，调用condition的await方法时，锁也会释放，在await返回之前，锁会被重新获取，并且锁计数会恢复到调用await方法时的值。
- 3，如果一个线程在等待的过程中被中断了，那么等待就会结束，并抛出interruptedexception异常，线程的中断标志位会被清理。
- 4，等待的线程以fifo的顺序被唤醒。
- 5，从await方法返回的线程们的获取到锁的顺序，和线程最开始获取锁的顺序相同，这是未指定情况下的默认实现。但是，公平锁更钟爱那些已经等待了最长时间的线程。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
|
/**
* returns a {@link condition} instance for use with this
* {@link lock} instance.
* <p>the returned {@link condition} instance supports the same
* usages as do the {@link object} monitor methods ({@link
* object#wait() wait}, {@link object#notify notify}, and {@link
* object#notifyall notifyall}) when used with the built-in
* monitor lock.
* <ul>
* <li>if this lock is not held when any of the {@link condition}
* {@linkplain condition#await() waiting} or {@linkplain
* condition#signal signalling} methods are called, then an {@link
* illegalmonitorstateexception} is thrown.
* <li>when the condition {@linkplain condition#await() waiting}
* methods are called the lock is released and, before they
* return, the lock is reacquired and the lock hold count restored
* to what it was when the method was called.
* <li>if a thread is {@linkplain thread#interrupt interrupted}
* while waiting then the wait will terminate, an {@link
* interruptedexception} will be thrown, and the thread's
* interrupted status will be cleared.
* <li> waiting threads are signalled in fifo order.
* <li>the ordering of lock reacquisition for threads returning
* from waiting methods is the same as for threads initially
* acquiring the lock, which is in the default case not specified,
* but for <em>fair</em> locks favors those threads that have been
* waiting the longest.
* </ul>
* @return the condition object
**/
public condition newcondition() {
return sync.newcondition();
}
|
可重入锁还有一些其他的方法，这里就不一一介绍了。this is the end.
总结
以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，谢谢大家对服务器之家的支持。如果你想了解更多相关内容请查看下面相关链接
原文链接：https://blog.csdn.net/li_canhui/article/details/85006114