ReentrantLock分析
要了解ReentrantLock必须要首先理解一下AQS(AbstractQueuedSynchronizer)的大概的内部结构
aqs 内部维护了一个Node 类型的head,tail ;
head: 等待获取锁的首节点
tail: 等待获取锁的尾节点
Node是一个内部类；
构造函数 Node(Thread thread, Node mode) 创建Node的时候需要一个当前的线程和线程等待的模式
等待模式有2种： Share,Exclusive 模式
waitStatus： 节点的等待的状态有4种状态
1: canceld 线程的获取锁由于超时或interrupt取消了
-1：singal 后续的节点需要被唤醒
-2: condition 当前的节点在condition queue, 当某个时候状态被改变的(status 变为0)时候才会去condition race 获取锁
-3：propagate 下一个acquireShared无条件的传播
0： 默认的值
如下是Node的代码， 可以理解为在aqs内部维护了一个双向的链表， 每个Node都是一个线程+模式构成的等待着排队获取锁的节点；
通过AQS.addWaiter方法知道如果acquire失败那么会创建一个等待的节点，添加到tail后面然后设置新创建的node为tail;
head(Thread,Mode)---next(Thread,Mode)----.......----tail(Thread,Mode)
static final class Node {
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueuing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() { // Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
ReentrantLock 内部维护了一个Sync对象 ( Sync extends AbstractQueuedSynchronizer) 继承了抽象了aqs
通过构造函数传递值来创建公平/非公平锁
public ReentrantLock(boolean fair);
fair 为true的时候创建一个公平锁 sync = new FairSync();
fair 为false 的时候创建一个非公平锁 sync = new NonfairSync();
然后分析一下公平锁/非公平锁是如何加锁和释放锁的呢？
加锁
lock() --> sync.lock();
FairSync:
----FairSync.acquire(1)--->AQS.acuire(1)-->FairSync.tryAcuire(1)
NonFairSync:
----NonfairSync.lock()-->compareAndSetStatus 获取锁
-->acquire(1) 获取锁 --->AQS.acuire(1)-->NonfairSync.tryAcuire(1)-->Sync.nonfairTryAcqure
如上面的流程图可以分析到公平和非公平锁的最大的区别在于2个地方
1： FairSync 和 NonFairSync内部的lock方法
FairSync的lock方法是直接调用acquire(1)方法
NonfairSync的lock是做了一个判断：
如果compareAndSetState成功的那么直接设置当前线性为独占线程
否则就进行acquire(1)
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);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
/**
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;
}*/
}
2: tryAcquire方法不同
FairSync的tryAcquire 方法说先判断state如果是0 那么进行判断是否当前线程所在的节点前面是否有等待的（其实就是排队的获取锁）
并且cas state 成功就获取到锁， 如果当前的线程已经获取到了锁那么state ++ ;
NonfairSync 是调用的tryAcquire-->nonfairTryAcquire
直接通过compareAndSetState来获取锁 不考虑当前的节点之前是否有等待的节点了 （可以理解为插队获取锁）
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
acquire(1);
}
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
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;
}
}
释放锁
unlock -->AQS.release()-->Sync.tryRelease()
所以是否锁的动作都是一样的
是否锁的是如何操作的呢？
class AbstractQueuedSynchronizer{
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)
//当前的节点修改状态为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.
*/
//获取当前节点的下一个节点
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;
}
if (s != null)
//如果下一个节点不为空， 那么通过LockSupport方法将下个节点的线程唤醒
LockSupport.unpark(s.thread);
}
}
总结
ReentrantLock内部实现了公平锁和非公平锁；
公平锁好比是一个排队的有序的队列 一个一个的执行
非公平锁好比是一个队列，总是有那么几个不守规矩的人插队， 打破了有序的规则；