用锁实现同步的方式：
1、synchronized基于JVM 语法简单，是不公平锁
不同线程多次调用wait方法，notify方法唤醒时则随机唤醒一个。所以是不公平锁
2、jdk实现-juc包下的lock 可以实现高级功能(手动释放可能会死锁)
ASQ实现组件
CountDownLatch
1、让线程都准备好一起执行
2、某个线程需要等其他线程都执行完
Semaphore
同一时间段，一次执行数量
CyclicBarrier
ReentrantLock
多线程同步内部实现
wait/notify synchronized ReentrantLock......
自旋是空转过程 占用CPU
让出CPU方法：yield、sleep、park
LockSupport.park();
唤醒park的线程
LockSupport.unpark( t1 );
ReentrantLock类
在分析加锁过程之前，需要了解下ReentrantLock类
package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
import java.util.Collection;
/**
* 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.
*
* A {@code ReentrantLock} is owned 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}.
*
*
The constructor for this class accepts an optional
* fairness parameter. When set {@code true}, under
* contention, locks favor granting access to the longest-waiting
* thread. Otherwise this lock does not guarantee any particular
* access order. Programs using fair locks accessed by many threads
* may display lower overall throughput (i.e., are slower; often much
* slower) than those using the default setting, but have smaller
* variances in times to obtain locks and guarantee lack of
* starvation. Note however, that fairness of locks does not guarantee
* fairness of thread scheduling. Thus, one of many threads using a
* fair lock may obtain it multiple times in succession while other
* active threads are not progressing and not currently holding the
* lock.
* Also note that the untimed {@link #tryLock()} method does not
* honor the fairness setting. It will succeed if the lock
* is available even if other threads are waiting.
*
*
It is recommended practice to always immediately
* follow a call to {@code lock} with a {@code try} block, most
* typically in a before/after construction such as:
*
*
{@code
* class X {
* private final ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* lock.lock(); // block until condition holds
* try {
* // ... method body
* } finally {
* lock.unlock()
* }
* }
* }}
*
* In addition to implementing the {@link Lock} interface, this
* class defines a number of {@code public} and {@code protected}
* methods for inspecting the state of the lock. Some of these
* methods are only useful for instrumentation and monitoring.
*
*
Serialization of this class behaves in the same way as built-in
* locks: a deserialized lock is in the unlocked state, regardless of
* its state when serialized.
*
*
This lock supports a maximum of 2147483647 recursive locks by
* the same thread. Attempts to exceed this limit result in
* {@link Error} throws from locking methods.
*
* @since 1.5
* @author Doug Lea
*/
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
/** Synchronizer providing all implementation mechanics */
private final Sync sync;
/**
* Base of synchronization control for this lock. Subclassed
* into fair and nonfair versions below. Uses AQS state to
* represent the number of holds on the lock.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* Performs {@link Lock#lock}. The main reason for subclassing
* is to allow fast path for nonfair version.
*/
abstract void lock();
/**
* 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();
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;
}
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;
}
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
final ConditionObject newCondition() {
return new ConditionObject();
}
// Methods relayed from outer class
final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}
final boolean isLocked() {
return getState() != 0;
}
/**
* Reconstitutes the instance from a stream (that is, deserializes it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
/**
* Sync object for non-fair locks
*/
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);
}
}
/**
* Sync object for fair locks
*/
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;
}
}
/**
* 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();
}
/**
* Acquires the lock.
*
*
Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
*
If the current thread already holds the lock then the hold
* count is incremented by one and the method returns immediately.
*
*
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();
}
/**
* Acquires the lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
*
Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
*
If the current thread already holds this lock then the hold count
* is incremented by one and the method returns immediately.
*
*
If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of two things happens:
*
*
*
* - The lock is acquired by the current thread; or
*
*
- Some other thread {@linkplain Thread#interrupt interrupts} the
* current thread.
*
*
*
* If the lock is acquired by the current thread then the lock hold
* count is set to one.
*
*
If the current thread:
*
*
*
* - has its interrupted status set on entry to this method; or
*
*
- is {@linkplain Thread#interrupt interrupted} while acquiring
* the lock,
*
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* In this implementation, as this method is an explicit
* interruption point, preference is given to responding to the
* interrupt over normal or reentrant acquisition of the lock.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
/**
* Acquires the lock only if it is not held by another thread at the time
* of invocation.
*
*
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()} will
* 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).
*
*
If the current thread already holds this lock then the hold
* count is incremented by one and the method returns {@code true}.
*
*
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);
}
/**
* Acquires the lock if it is not held by another thread within the given
* waiting time and the current thread has not been
* {@linkplain Thread#interrupt interrupted}.
*
*
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. If this lock has been set to use a fair ordering policy then
* an available lock will not be acquired if any other threads
* are waiting for the lock. This is in contrast to the {@link #tryLock()}
* method. If you want a timed {@code tryLock} that does permit barging on
* a fair lock then combine the timed and un-timed forms together:
*
*
{@code
* if (lock.tryLock() ||
* lock.tryLock(timeout, unit)) {
* ...
* }}
*
* If the current thread
* already holds this lock then the hold count is incremented by one and
* the method returns {@code true}.
*
*
If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
*
*
*
* - The lock is acquired by the current thread; or
*
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
*
- The specified waiting time elapses
*
*
*
* If the lock is acquired then the value {@code true} is returned and
* the lock hold count is set to one.
*
*
If the current thread:
*
*
*
* - has its interrupted status set on entry to this method; or
*
*
- is {@linkplain Thread#interrupt interrupted} while
* acquiring the lock,
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
*
In this implementation, as this method is an explicit
* interruption point, preference is given to responding to the
* interrupt over normal or reentrant acquisition of the lock, and
* over reporting the elapse of the waiting time.
*
* @param timeout the time to wait for the lock
* @param unit the time unit of the timeout argument
* @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} if the waiting time elapsed before
* the lock could be acquired
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if the time unit is null
*/
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
/**
* Attempts to release this lock.
*
*
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);
}
/**
* Returns a {@link Condition} instance for use with this
* {@link Lock} instance.
*
*
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.
*
*
*
* - 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.
*
*
- 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.
*
*
- 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.
*
*
- Waiting threads are signalled in FIFO order.
*
*
- 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 fair locks favors those threads that have been
* waiting the longest.
*
*
*
* @return the Condition object
*/
public Condition newCondition() {
return sync.newCondition();
}
/**
* Queries the number of holds on this lock by the current thread.
*
* A thread has a hold on a lock for each lock action that is not
* matched by an unlock action.
*
*
The hold count information is typically only used for testing and
* debugging purposes. For example, if a certain section of code should
* not be entered with the lock already held then we can assert that
* fact:
*
*
{@code
* class X {
* ReentrantLock lock = new ReentrantLock();
* // ...
* public void m() {
* assert lock.getHoldCount() == 0;
* lock.lock();
* try {
* // ... method body
* } finally {
* lock.unlock();
* }
* }
* }}
*
* @return the number of holds on this lock by the current thread,
* or zero if this lock is not held by the current thread
*/
public int getHoldCount() {
return sync.getHoldCount();
}
/**
* Queries if this lock is held by the current thread.
*
* Analogous to the {@link Thread#holdsLock(Object)} method for
* built-in monitor locks, this method is typically used for
* debugging and testing. For example, a method that should only be
* called while a lock is held can assert that this is the case:
*
*
{@code
* class X {
* ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* assert lock.isHeldByCurrentThread();
* // ... method body
* }
* }}
*
* It can also be used to ensure that a reentrant lock is used
* in a non-reentrant manner, for example:
*
*
{@code
* class X {
* ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* assert !lock.isHeldByCurrentThread();
* lock.lock();
* try {
* // ... method body
* } finally {
* lock.unlock();
* }
* }
* }}
*
* @return {@code true} if current thread holds this lock and
* {@code false} otherwise
*/
public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
}
/**
* Queries if this lock is held by any thread. This method is
* designed for use in monitoring of the system state,
* not for synchronization control.
*
* @return {@code true} if any thread holds this lock and
* {@code false} otherwise
*/
public boolean isLocked() {
return sync.isLocked();
}
/**
* Returns {@code true} if this lock has fairness set true.
*
* @return {@code true} if this lock has fairness set true
*/
public final boolean isFair() {
return sync instanceof FairSync;
}
/**
* Returns the thread that currently owns this lock, or
* {@code null} if not owned. When this method is called by a
* thread that is not the owner, the return value reflects a
* best-effort approximation of current lock status. For example,
* the owner may be momentarily {@code null} even if there are
* threads trying to acquire the lock but have not yet done so.
* This method is designed to facilitate construction of
* subclasses that provide more extensive lock monitoring
* facilities.
*
* @return the owner, or {@code null} if not owned
*/
protected Thread getOwner() {
return sync.getOwner();
}
/**
* Queries whether any threads are waiting to acquire this lock. Note that
* because cancellations may occur at any time, a {@code true}
* return does not guarantee that any other thread will ever
* acquire this lock. This method is designed primarily for use in
* monitoring of the system state.
*
* @return {@code true} if there may be other threads waiting to
* acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* Queries whether the given thread is waiting to acquire this
* lock. Note that because cancellations may occur at any time, a
* {@code true} return does not guarantee that this thread
* will ever acquire this lock. This method is designed primarily for use
* in monitoring of the system state.
*
* @param thread the thread
* @return {@code true} if the given thread is queued waiting for this lock
* @throws NullPointerException if the thread is null
*/
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
}
/**
* Returns an estimate of the number of threads waiting to
* acquire this lock. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring of the system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
}
/**
* Returns a collection containing threads that may be waiting to
* acquire this lock. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
protected Collection getQueuedThreads() {
return sync.getQueuedThreads();
}
/**
* Queries whether any threads are waiting on the given condition
* associated with this lock. Note that because timeouts and
* interrupts may occur at any time, a {@code true} return does
* not guarantee that a future {@code signal} will awaken any
* threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this lock. Note that because
* timeouts and interrupts may occur at any time, the estimate
* serves only as an upper bound on the actual number of waiters.
* This method is designed for use in monitoring of the system
* state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this lock.
* Because the actual set of threads may change dynamically while
* constructing this result, the returned collection is only a
* best-effort estimate. The elements of the returned collection
* are in no particular order. This method is designed to
* facilitate construction of subclasses that provide more
* extensive condition monitoring facilities.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
protected Collection getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes either the String {@code "Unlocked"}
* or the String {@code "Locked by"} followed by the
* {@linkplain Thread#getName name} of the owning thread.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
}
ReentrantLock 含有一个实现了AbstractQueuedSynchronizer的Sync内部类。Sync的abstract抽象类实现有FairSync(公平锁)、NonfairSync(非公平锁)
ASQ类结构
包含的属性字段head(头节点)、tail(尾结点)、state和内部类node节点
package java.util.concurrent.locks;
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
protected AbstractQueuedSynchronizer() { }
//Node节点
static final class Node {
static final Node SHARED = new Node();
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;
volatile int waitStatus;
volatile Node prev;
volatile Node next;
volatile Thread thread;
Node nextWaiter;
final boolean isShared() {
return nextWaiter == SHARED;
}
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;
}
}
//头节点
private transient volatile Node head;
//尾结点
private transient volatile Node tail;
//是否可用状态值
private volatile int state;
ReentrantLock加锁过程
构造方法传true则创建公平锁FairSync
ReentrantLock调用lock方法加锁
调用到公平锁FairSync的lock方法
公平锁加锁FairSync.lock方法
AQS的acquire方法
AbstractQueuedSynchronizer#acquire
首先调用tryAcquire方法尝试加锁，由于这个方法取反。只有tryAcquire方法返回false，取反返回true才会继续执行&&之后的方法。addWaiter()方法把当前线程添加AQS的NODE节点中。acquireQueued尝试队列
tryAcquire尝试加锁
FairSync#tryAcquire
current属性，获取当前线程。首先获取AQS的state，是否可用。对于第一次加锁的t1线程，此时默认是0表示可用状态。c=0，
对于t1线程调用hasQueuedPredecessors方法
tail和head此时都为null，所以h=t，方法返回false，取反返回true。调用compareAndSetState，把state改为acquires的值1，并返回true，加锁成功。getExclusiveOwnerThread方法表示是重入锁。
addWaiter()
到这里t1线程加锁成功后，并没有解锁,一直休眠，t2线程加锁。
tryAcquire方法，此时state值是1被t1线程持有，getState()的值是1，返回false。
调用addWaiter(Node.EXCLUSIVE)方法
根据currentThread，mode是null。创建node节点。这时，ASQ还没有数据，tail节点为null。进入enq(node);方法node的值是t2创建的节点
创建的初始化节点，head和tail都是这个空的Node节点
这里是个死循环，首先判断tail尾节点是否为null，如果是null则进行CAS操作初始化赋值head头结点、tail尾结点为new Node()。第二次循环则tail不为null。
步骤1，node节点的prev指向初始节点。步骤2，CAS操作node节点设置为tail节点。步骤3，初始节点的next指向当前node节点
如果此时t1和t2线程都没有解锁，t3线程进入则插入队列过程。首先把tail节点赋值给t变量，然后执行t3-1、t3-2、t3-1与1、2、3相同
acquireQueued方法
队列插入完成，调用acquireQueued方法。参数是当前节点Node也是tail指向的最后一个节点
也是一个死循环。把当前节点的前一个节点赋值给p变量
如果p节点时head，可以调用tryAcquire方法，尝试加锁。如果t1线程休眠完成且释放了锁，t2线程将会拿到锁，进入方法，
setHead方法，对应sethead-1、sethead-2、sethead-3。
p.next = null，此时head指向第二个thread和prev都为null的节点并返回
如果此时node是t3线程进入acquireQueued方法。则p不等于head。
调用shouldParkAfterFailedAcquire方法
当然，t2线程如果尝试加锁没有成功也会进入这里。首先判断waitStatus值，默认是0。直接进入else方法，CAS操作把前一个节点(p)的waitStatus值设为Node.SIGNAL(-1)，并返回false。由于acquireQueued方法是一个死循环，所以第二次t2线程进入时，如果此时t1线程还没有释放，还会进入shouldParkAfterFailedAcquire方法。此时p的waitStatus值已经被上一次循环赋值为-1，所以直接返回true。将会调用parkAndCheckInterrupt方法，当前线程让出CPU并休眠
对于t3线程也是经历相同的步骤，首先判断之前的节点是否是head节点。然后在调用shouldParkAfterFailedAcquire方法对之前的节点的waitStatus赋值-1，并休眠当前线程。
到这里加锁过程基本完成
非公平锁加锁NonfairSync.lock()
对于非公平锁加锁过程
首先尝试获取锁，如果失败，则执行公平锁逻辑
非公平锁的tryAcquire方法并没有调用hasQueuedPredecessors方法，如果有在排队，且不是当前线程将不会尝试加锁
总结：
1、ReentrantLock结构的属性及其内部类实现
2、AQS的结构属性，AQS在ReentrantLock的应用
3、加锁的初始化和入队过程及其结构图
4、对于公平锁，锁释放后那个线程会获取
5、非公平锁与公平锁的加锁过程的区别
head节点的Thread=null，队列中的head是线程持有锁的Node