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手写AQS非公平锁的示例分析，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
1. Unsafe工具类
package com.shi.flink.unsafeTest;
import sun.misc.Unsafe;
import java.lang.reflect.Field;
/**
* @author shiye
* @create 2021-03-30 17:03
*/
public class UnsafeUtil {
public static Unsafe getInstance() {
Field field = null;
try {
field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
return (Unsafe) field.get(null);
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
return null;
}
}2. 手写AQS抽象类
package com.shi.flink.shilock;
import com.shi.flink.unsafeTest.UnsafeUtil;
import sun.misc.Unsafe;
import java.util.concurrent.locks.AbstractOwnableSynchronizer;
import java.util.concurrent.locks.LockSupport;
/**
* 自己写抽象AQS实现
*
* @author shiye
* @create 2021-03-30 14:10
*/
public abstract class ShiAQS extends AbstractOwnableSynchronizer implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
/**
* 头指针
*/
private transient volatile Node head;
/**
* 尾指针
*/
private transient volatile Node tail;
/**
* 状态值：
* 0：空闲，
* 1：正在有人使用
*/
private volatile int state;
/**
* 获取当前状态
*
* @return
*/
protected final int getState() {
return state;
}
/**
* 设置当前锁的状态
*
* @param state
*/
public void setState(int state) {
this.state = state;
}
/**
* 使用unsafe类来初始化一些参数值
*/
private static final Unsafe unsafe = UnsafeUtil.getInstance();
private static long stateOffset;
private static long headOffset;
private static long tailOffset;
private static long waitStatusOffset;
private static long nextOffset;
static {
try {
stateOffset = unsafe.objectFieldOffset(ShiAQS.class.getDeclaredField("state"));
headOffset = unsafe.objectFieldOffset(ShiAQS.class.getDeclaredField("head"));
tailOffset = unsafe.objectFieldOffset(ShiAQS.class.getDeclaredField("tail"));
waitStatusOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("waitStatus"));
nextOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("next"));
} catch (NoSuchFieldException e) {
e.printStackTrace();
}
}
/**
* 设置状态
*
* @param expect
* @param update
* @return
*/
protected boolean compareAndSetState(int expect, int update) {
//读取传入对象o在内存中偏移量为offset位置的值与期望值expected作比较
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
/**
* 设置头指针
*
* @param update
* @return
*/
private final boolean compareAndSetHead(Node update) {
return unsafe.compareAndSwapObject(this, headOffset, null, update);
}
/**
* 如果pre节点得waitStatus值为ws，
* 则把signal赋值给waitStatus
*
* @param pre
* @param ws
* @param signal
* @return
*/
private static boolean compareAndSetWaitStatus(Node pre, int ws, int signal) {
return unsafe.compareAndSwapInt(pre, waitStatusOffset, ws, signal);
}
/**
* 设置尾指针
*
* @param expect
* @param update
* @return
*/
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}
/**
* 设置下一个节点
*
* @param node
* @param expect
* @param update
* @return
*/
private static final boolean compareAndSetNext(Node node,
Node expect,
Node update) {
return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
}
/**
* 解锁方法
*
* @param arg
*/
protected void release(int arg) throws Exception {
//尝试去释放占用锁得线程
boolean tryRelease = tryRelease(arg);
if (tryRelease) {
Node h = head;
if (h != null && h.waitStatus != 0) {
unparkSuccessor(h);
}
}
}
/**
* 尝试去释放占用锁得线程
*
* @param arg
* @return
* @throws Exception
*/
protected boolean tryRelease(int arg) throws Exception {
if (Thread.currentThread() != getExclusiveOwnerThread()) {
//如果当前线程不是占用锁得线程就抛出异常
throw new Exception("解锁失败，当前线程不是占用锁得线程无法解锁");
} else {
setExclusiveOwnerThread(null);
this.setState(0);
return true;
}
}
/**
* 打断某个线程
*
* @return
*/
protected boolean interruptThread(Thread thread) throws Exception {
Thread ownerThread = getExclusiveOwnerThread();
if (ownerThread == thread) {
//如果是正在运行得线程
compareAndSetState(1, 0);
setExclusiveOwnerThread(null);
} else if (head != null) {
//再对类中查找当前线程，并且取消排队
for (Node next1 = head.next; next1 != null; next1 = next1.next) {
if (next1.thread == thread) {
compareAndSetWaitStatus(next1, next1.waitStatus, 1);
}
}
}
//解锁
thread.interrupt();
System.out.println(thread.getName() + " 已经中断了 ====> ");
unparkSuccessor(head);
System.out.println(thread.getName() + " 已经结束了 ====> ");
return false;
}
/**
* 自定义一个内部类Node节点
*/
static final class Node {
//共享模式标记
static final Node shared = new Node();
//独占锁标记
static final Node excusive = null;
//waitStatus值，指示线程已取消
static final int cancelled = 1;
//waitStatus值，用于指示后续线程需要解除等待状态
static final int signal = -1;
//waitStatus值，指示线程正在等待条件
static final int condition = -2;
//waitStatus值，指示下一个acquireShared应该 无条件传播
static final int propagate = -3;
//锁的等待状态
volatile int waitStatus;
//前指针
volatile Node prev;
//后指针
volatile Node next;
//线程
volatile Thread thread;
Node nextWaiter;
//是否是共享锁
final boolean isShared() {
return nextWaiter == shared;
}
//无参构造
public Node() {
}
public Node(Node nextWaiter, Thread thread) {
this.nextWaiter = nextWaiter;
this.thread = thread;
}
/**
* 获取前节点
*
* @return
*/
public Node getPrev() {
Node p = prev;
if (p == null) {
throw new NullPointerException("前节点不能为空");
}
return p;
}
}
/**
* 获得
*
* @param arg
*/
public void acquire(int arg) {
//1.尝试去排队
boolean tryAcquire = tryAcquire(arg);
if (!tryAcquire) {
//2.如果抢占锁失败,就去排队
Node node = addWaiter(Node.excusive);
//3.对已经再队列中的节点，进行休眠等侯
acquireQueued(node, arg);
}
}
/**
* 先尝试去排队
* 1.先获取锁得状态，如果状态为0，就尝试去占用一次锁
* 否则返回占用失败
*
* @param arg
* @return true:表示抢占锁成功
* false:表示抢占所失败
*/
public final boolean tryAcquire(int arg) {
Thread current = Thread.currentThread();
int state = getState();
if (state == 0) {
//如果空闲了,就尝试去占用一次锁
if (compareAndSetState(0, arg)) {
//抢占成功就返回true,并设置线程
setExclusiveOwnerThread(current);
return true;
}
} else if (getExclusiveOwnerThread() == current) {
//如果当前当前线程多次抢占锁,就将状态+arg
int nextState = state + arg;
if (nextState < 0) {
throw new Error("超过最大锁计数");
}
setState(nextState);
return true;
}
return false;
}
/**
* 添加等待队列
*
* @param mode
*/
public Node addWaiter(Node mode) {
Node node = new Node(mode, Thread.currentThread());
Node temp = tail;
if (temp == null) {
//入队
enQueue(node);
return node;
} else {
//如果队列中不为空，就把当前节点添加到尾节点中
node.prev = temp;
if (compareAndSetTail(temp, node)) {
temp.next = node;
return node;
}
}
return node;
}
/**
* 入队
* 把node节点添加到队列中，
* 如果队列为null就初始化一个队列并且把node节点添加到尾节点中
*
* @param node 返回当前节点
*/
public Node enQueue(Node node) {
while (true) {
Node temp = tail;
if (temp == null) {
//创建一个头指针
compareAndSetHead(new Node());
//让尾指针也指向头指针(空节点)
tail = head;
} else {
node.prev = temp;
if (compareAndSetTail(temp, node)) {
temp.next = node;
return node;
}
}
}
}
/**
* @param node 当前正在侯队中得节点
* @param arg
* @return
*/
protected boolean acquireQueued(Node node, int arg) {
boolean failed = true;
try {
//是否被打断，默认false
boolean interrupted = false;
while (true) {
final Node p = node.getPrev();
if (p == head && tryAcquire(arg)) {
//如果是他的头节点是head，并且尝试抢占锁成功就出队,让当前线程运行
setHead(node);
p.next = null;//利于gc回收
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) {
//pre节点得waitStatus 设置成-1，并且让当前线程阻塞，打断当前线程
interrupted = true;
}
}
} finally {
if (failed) cancelAcquire(node);
}
}
protected final void cancelAcquire(Node node) {
if (node == null) return;
node.thread = null;
Node pre = node.prev;
while (pre.waitStatus > 0) {
node.prev = pre = pre.prev;
}
Node predNext = pre.next;
node.waitStatus = Node.cancelled;
if (node == tail && compareAndSetTail(node, pre)) {
compareAndSetNext(pre, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
int ws;
if (pre != head &&
((ws = pre.waitStatus) == Node.signal ||
(ws <= 0 && compareAndSetWaitStatus(pre, ws, Node.signal))) &&
pre.thread != null) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pre, predNext, next);
} else {
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
/**
* 解锁必须成功
*
* @param node
*/
protected final void unparkSuccessor(Node node) {
int ws = node.waitStatus;
if (ws < 0) {
compareAndSetWaitStatus(node, ws, 0);
}
/**
* AQS源码是这样实现得
* 如果当前节点不为空，并且用户取消了，就从尾节点往前遍历一个，直到找到最前面得一个节点，解锁当前线程
*/
Node next1 = node.next;
// if (next1 == null || next1.waitStatus > 0) {
// next1 = null;
// for (Node t = tail; t != null && t != node; t = t.prev) {
// if (t.waitStatus <= 0) {
// next1 = t;
// }
// }
// }
/**
* 我自己实现，从前往后找
*/
if (next1 != null && next1.waitStatus > 0) {
for (next1 = next1.next; next1 != null; next1 = next1.next) {
if (next1.waitStatus <= 0) {
break;
}
}
}
if (next1 != null) {
//唤醒下一个线程
System.out.println(next1.thread.getName() + " 开始唤醒了 ====> ");
LockSupport.unpark(next1.thread);
System.out.println(next1.thread.getName() + " 已经唤醒了 ====> ");
}
}
/**
* 将pre节点得waitStatus 设置成-1
*
* @param pre
* @param node
* @return
*/
protected static boolean shouldParkAfterFailedAcquire(Node pre, Node node) {
//获取node节点得前一个节点得状态
int ws = pre.waitStatus;
//如果是-1 就返回true
if (ws == Node.signal) {
return true;
}
if (ws > 0) {
do {
pre = pre.prev;
node.prev = pre;
} while (pre.waitStatus > 0);
pre.next = node;
} else {
//设置成-1
boolean flag = compareAndSetWaitStatus(pre, ws, Node.signal);
// System.out.println("设置成-1是否成功：" + flag);
}
return false;
}
/**
* 阻塞当前线程，并且返回当前线程得打断状态
*
* @return true: 打断线程成功
*/
protected final boolean parkAndCheckInterrupt() {
//打断线程，让线程阻塞
LockSupport.park(this);
return Thread.interrupted();
}
public Node getHead() {
return head;
}
public void setHead(Node head) {
this.head = head;
}
}3.非公平所实现
package com.shi.flink.shilock;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
/**
* 自定义非公平锁
* @author shiye
* @create 2021-03-30 11:02
*/
public class ShiNonfairLock extends ShiAQS implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
@Override
public void lock() {
if(compareAndSetState(0, 1)){
//如果抢到了锁，就把当前线程设置进去
setExclusiveOwnerThread(Thread.currentThread());
}else{
// 否则就去排队
acquire(1);
}
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return false;
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
try {
super.release(1);
} catch (Exception e) {
e.printStackTrace();
}
}
@Override
public Condition newCondition() {
return null;
}
/**
* 打断某个线程(自己瞎写的，有bug)
* @param thread
* @return
*/
public boolean interruptThread(Thread thread) throws Exception {
return super.interruptThread(thread);
}
}4.测试
package com.shi.flink.shilock;
import java.util.concurrent.TimeUnit;
/**
* @author shiye
* @create 2021-03-31 17:09
*/
public class MyLockTest {
public static void main(String[] args) throws Exception {
ShiNonfairLock lock = new ShiNonfairLock();
new Thread(() -> {
try {
System.out.println("A 线程进入到...加锁过程");
lock.lock();
System.out.println("A 已经抢占到锁...休眠10s后运行......");
TimeUnit.SECONDS.sleep(10);
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("A线程运行完成，开始解锁....");
lock.unlock();
}
}, "A").start();
TimeUnit.SECONDS.sleep(1);
Thread B = new Thread(() -> {
try {
System.out.println("B 线程进入到...加锁过程");
lock.lock();
System.out.println("B 已经抢占到锁...休眠10s后运行......");
TimeUnit.SECONDS.sleep(10);
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("B线程运行完成，开始解锁....");
lock.unlock();
}
}, "B");
B.start();
TimeUnit.SECONDS.sleep(1);
new Thread(() -> {
try {
System.out.println("C 线程进入到...加锁过程");
lock.lock();
System.out.println("C 已经抢占到锁...休眠10s后运行......");
TimeUnit.SECONDS.sleep(10);
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("C线程运行完成，开始解锁....");
lock.unlock();
}
}, "C").start();
TimeUnit.SECONDS.sleep(1);
new Thread(() -> {
try {
System.out.println("D 线程进入到...加锁过程");
lock.lock();
System.out.println("D 已经抢占到锁...休眠10s后运行......");
TimeUnit.SECONDS.sleep(10);
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("D线程运行完成，开始解锁....");
lock.unlock();
}
}, "D").start();
// TimeUnit.SECONDS.sleep(1);
// System.out.println("强制让 " + B.getName() + " 线程中断...");
// lock.interruptThread(B);
}
}看完上述内容，你们掌握手写AQS非公平锁的示例分析的方法了吗？如果还想学到更多技能或想了解更多相关内容，欢迎关注亿速云行业资讯频道，感谢各位的阅读！
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