Lock之ReentrantLock及实现生产者消费者和死锁
Lock是顶层接口，它的实现逻辑并未用到synchronized，而是利用了volatile的可见性。ReentrantLock对了Lock接口的实现主要依赖了Sync，而Sync继承了
AbstractQueuedSynchronizer(AQS)
ReentrantLock:
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 this lock instance from a stream.
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
}
在AQS中，定义了一个volatile int state 变量作为共享资源。
/**
* The synchronization state.
*/
private volatile int state;
如果线程获取此共享资源失败，则进入同步FIFO队列中等待；如果成功获取资源就执行临界区代码。执行完释放资源时，会通知同步队列中的等待线程来获取资源后出对并执行。
看其lock方法：
首先默认构造是非公平锁，所谓的公平锁就是先等待的线程先获得锁
public ReentrantLock() {
sync = new NonfairSync();
}
NonfairSync：
/**
* 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)) //compareAndSetState(int expect, int update)
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
} protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
ReentrantLock：
public void lock() {
sync.lock();
}
所以，默认执行的是NonfairSync中的lock()实现，利用Unsafe类的CAS，期望state值为0时将其值设为1，返回是否成功
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
因此ReentrantLock的lock()方法只有在state为0时才能获得锁，并将state设为1。这样其他线程就无法获取锁，只能等待。
由于ReentrantLock是可重入锁，即在获得锁的情况下，可以再次获得锁。并且线程可以进入任何一个它已经拥有的锁所同步着的代码块。若在没有释放锁的情况下，
再次获得锁，则state加1，在释放资源时，state减1，因此Lock获取多少次锁就要释放多少次锁，直到state为0。
Conditon中的await()对应Object的wait()，Condition中的signal()对应Object的notify()，Condition中的signalAll()对应Object的notifyAll()
两个线程交替执行例子（同理生产者消费者也是这样交替执行）：
package com.yang.spbo.other.lock; import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock; /**
* A,B两个线程交替执行
* 〈功能详细描述〉
*
* @author 17090889
* @see [相关类/方法]（可选）
* @since [产品/模块版本] （可选）
*/
public class ConditionService {
private ReentrantLock lock = new ReentrantLock();
/**
* 两个线程所以创建两个condition
*/
private Condition A = lock.newCondition();
private Condition B = lock.newCondition(); private int number = 1; private boolean flag = false; private void executeA() {
while (number < 100) {
try {
lock.lock();
if (!flag) {
System.out.println("A等待");
A.await();
}
System.out.println("A " + number);
number++;
flag = false;
System.out.println("B唤醒");
B.signal();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
} } private void executeB() {
while (number < 100) {
try {
lock.lock();
if (flag) {
System.out.println("B等待");
B.await();
}
System.out.println("B " + number);
number++;
flag = true;
System.out.println("A唤醒");
A.signal();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
} } public static void main(String[] args) {
final ConditionService cs = new ConditionService();
ThreadPoolExecutor executor = new ThreadPoolExecutor(2, 5, 1, TimeUnit.MINUTES, new LinkedBlockingDeque<Runnable>(1000), new MyThreadFactory("conditionService")); executor.execute(new Runnable() {
@Override
public void run() {
System.out.println(Thread.currentThread().getName());
cs.executeA();
}
});
executor.execute(new Runnable() {
@Override
public void run() {
System.out.println(Thread.currentThread().getName());
cs.executeB();
}
});
} static class MyThreadFactory implements ThreadFactory {
/**
* 线程名字前缀
*/
private String namePrefix; private AtomicInteger id = new AtomicInteger(1); public MyThreadFactory(String namePrefix) {
this.namePrefix = namePrefix;
} @Override
public Thread newThread(Runnable r) {
String threadName = namePrefix + "-worker-" + id.getAndIncrement();
Thread thread;
thread = new Thread(r, threadName);
return thread;
}
}
}
运行结果：
conditionService-worker-1
A等待
conditionService-worker-2
B 1
A唤醒
B等待
A 2
B唤醒
A等待
B 3
A唤醒
B等待
A 4
B唤醒
A等待
B 5
A唤醒
B等待
A 6
ReentrantLock实现死锁：
/**
* ReentrantLock实现死锁
* 〈功能详细描述〉
*
* @author 17090889
* @see [相关类/方法]（可选）
* @since [产品/模块版本] （可选）
*/
public class DeadLockTest {
private static ReentrantLock lock1 = new ReentrantLock();
private static ReentrantLock lock2 = new ReentrantLock(); public static void main(String[] args) {
new Thread(new Runnable() {
@Override
public void run() {
lock1.lock();
try {
Thread.sleep(1000);
lock2.lock();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock1.unlock();
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
lock2.lock();
try {
Thread.sleep(1000);
lock1.lock();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock2.unlock();
} }
}).start();
}
}
7996 Main
3960 Jps
8396 Launcher
5568 Launcher
1204
8212 Bootstrap
396 ProfilerServer
=============================
"Thread-1":
waiting for ownable synchronizer 0x00000007d5e6f5c0, (a java.util.concurrent.l
ocks.ReentrantLock$NonfairSync),
which is held by "Thread-0"
"Thread-0":
waiting for ownable synchronizer 0x00000007d5e6f5f0, (a java.util.concurrent.l
ocks.ReentrantLock$NonfairSync),
which is held by "Thread-1"
Java stack information for the threads listed above:
===================================================
"Thread-1":
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000007d5e6f5c0> (a java.util.concurrent.lock
s.ReentrantLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInt
errupt(AbstractQueuedSynchronizer.java:834)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(A
bstractQueuedSynchronizer.java:867)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(Abstrac
tQueuedSynchronizer.java:1197)
at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLo
ck.java:214)
at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:290)
at deadLock.DeadLockTest$2.run(DeadLockTest.java:38)
at java.lang.Thread.run(Thread.java:745)
"Thread-0":
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000007d5e6f5f0> (a java.util.concurrent.lock
s.ReentrantLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInt
errupt(AbstractQueuedSynchronizer.java:834)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(A
bstractQueuedSynchronizer.java:867)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(Abstrac
tQueuedSynchronizer.java:1197)
synchronized实现死锁:
/**
* synchronized实现死锁
* 〈功能详细描述〉
*
* @author 17090889
* @see [相关类/方法]（可选）
* @since [产品/模块版本] （可选）
*/
public class DeadLockTest2 {
private static Object obj1 = new Object();
private static Object obj2 = new Object(); public static void main(String[] args) {
new Thread(new Runnable() {
@Override
public void run() {
synchronized (obj1) {
System.out.println("thead1 get lock1");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (obj2) {
System.out.println("thead1 get lock2");
}
System.out.println("thread1 end");
}
}
}, "thead1").start();
new Thread(new Runnable() {
@Override
public void run() {
synchronized (obj2) {
System.out.println("thead2 get lock2");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (obj1) {
System.out.println("thead2 get lock1");
}
System.out.println("thread2 end");
}
}
}, "thead2").start();
}
}
=============================
"thead2":
waiting to lock monitor 0x000000000b15a7c8 (object 0x00000007d5e6f290, a java.
lang.Object),
which is held by "thead1"
"thead1":
waiting to lock monitor 0x000000000b1593d8 (object 0x00000007d5e6f2a0, a java.
lang.Object),
which is held by "thead2"
Java stack information for the threads listed above:
===================================================
"thead2":
at deadLock.DeadLockTest2$2.run(DeadLockTest2.java:44)
- waiting to lock <0x00000007d5e6f290> (a java.lang.Object)
- locked <0x00000007d5e6f2a0> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:745)
"thead1":
at deadLock.DeadLockTest2$1.run(DeadLockTest2.java:27)
- waiting to lock <0x00000007d5e6f2a0> (a java.lang.Object)
- locked <0x00000007d5e6f290> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:745)
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