第47天打卡学习（单例模式 深入了解CAS 原子引用 各种锁的理解）
饿汉式 DCL懒汉模式 探究！
饿汉式
package com.kuang.single;
//饿汉式单例
//单例模式重要思想是构造器私有
public class Hungry {
//可能会浪费空间
private byte[] data1 = new byte[1024*1024];
private byte[] data2 = new byte[1024*1024];
private byte[] data3 = new byte[1024*1024];
private byte[] data4 = new byte[1024*1024];
private Hungry(){
}
private final static Hungry HUNGRY = new Hungry();
public static Hungry getInstance(){
return HUNGRY;
}
}
DCL懒汉式
package com.kuang.single;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
//懒汉式单例
public class LayzMan {
private static boolean qinjiang = false;
private LayzMan() {
synchronized (LayzMan.class){
if (qinjiang == false){
qinjiang = true;
}
else {
throw new RuntimeException("不要试图使用反射破坏异常");
}
}
}
private volatile static LayzMan layzMan;
public static LayzMan getInstance() {
//加锁
//双重检测锁模式的 懒汉式单例 DCL懒汉模式
if (layzMan == null) {
synchronized (LayzMan.class) {//锁class只有一个
if (layzMan == null) {
layzMan = new LayzMan();//不是一个原子性操作
}
}
}
return layzMan;//加了线程B后 此时layzMan还没有完成构造
}
/**
*1.分配内存空间
* 2.执行构造方法，初始化对象
* 3.把这个对象指向这个空间
*
* 假设原本希望执行123
* 真实可能执行132 若只有单线程A可以执行 若再加了一个线程B会出现问题 了线程B后 此时layzMan还没有完成构造
*/
//反射
public static void main(String[] args) throws Exception {
// LayzMan instance = LayzMan.getInstance();
Field qinjiang = LayzMan.class.getDeclaredField("qinjiang");
qinjiang.setAccessible(true);
Constructor<LayzMan> declaredConstructor = LayzMan.class.getDeclaredConstructor(null);
declaredConstructor.setAccessible(true);
LayzMan instance = declaredConstructor.newInstance();
qinjiang.set(instance,false);
LayzMan instance2 = declaredConstructor.newInstance();
System.out.println(instance);
System.out.println(instance2);
}
}
静态内部类
package com.kuang.single;
//静态内部类 在一个类里面再写一个静态类
public class Holder {
private Holder(){
}
public static Holder getInstance(){
return InnerClass.HOLDER;
}
public static class InnerClass{
private static final Holder HOLDER = new Holder();
}
}
单例不安全，存在反射
枚举enum
package com.kuang.single;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
//enum （枚举）是个什么？ 枚举本身也是一个Class类
//反射不能破坏枚举的单例Cannot reflectively create enum objects
public enum EnumSingle {
INSTANCE;
public EnumSingle getInstance(){
return INSTANCE;
}
}
class Test{
public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
EnumSingle instance1 = EnumSingle.INSTANCE;
Constructor<EnumSingle> declaredConstructor = EnumSingle.class.getDeclaredConstructor(String.class,int.class);
declaredConstructor.setAccessible(true);
EnumSingle instance2 = declaredConstructor.newInstance();
System.out.println(instance1);
System.out.println(instance2);
//NoSuchMethodException: com.kuang.single.EnumSingle.<init>()这个类里面没有空参构造器
}
}
枚举为有参构造 两个参数
把一个class文件反编译为java的操作：
枚举类型的最终反编译源码：
// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3)
// Source File Name: EnumSingle.java
package com.kuang.single;
public final class EnumSingle extends Enum
{
public static EnumSingle[] values()
{
return (EnumSingle[])$VALUES.clone();
}
public static EnumSingle valueOf(String name)
{
return (EnumSingle)Enum.valueOf(com/kuang/single/EnumSingle, name);
}
private EnumSingle(String s, int i)
{
super(s, i);
}
public EnumSingle getInstance()
{
return INSTANCE;
}
public static final EnumSingle INSTANCE;
private static final EnumSingle $VALUES[];
static
{
INSTANCE = new EnumSingle("INSTANCE", 0);
$VALUES = (new EnumSingle[] {
INSTANCE
});
}
}
19深入理解CAS
什么是CAS
Unsafe类
package com.cas;
import java.util.concurrent.atomic.AtomicInteger;
public class CASDemo {
//CAS compareAndSet比较并交换
public static void main(String[] args) {
AtomicInteger atomicInteger = new AtomicInteger(2021);
//expect:期望 update：更新
// public final boolean compareAndSet(int expect, int update)
//如果我期望的值达到了，那么就更新，否则就不更新,CAS是CPU的并发原语
System.out.println(atomicInteger.compareAndSet(2021, 2022));
System.out.println(atomicInteger.get());
atomicInteger.getAndIncrement();//
System.out.println(atomicInteger.compareAndSet(2021, 2022));
System.out.println(atomicInteger.get());
}
}
CAS：比较当前工作内存中的值和主内存中的值，如果这个值是期望的，那么则执行操作！如果不是就一直循环！
缺点：
1.循环会耗时
2.一次性只能保证一个共享变量的原子性
3.存在ABA问题
CAS:ABA问题：（狸猫换太子）
package com.cas;
import java.util.concurrent.atomic.AtomicInteger;
public class CASDemo {
//CAS compareAndSet比较并交换
public static void main(String[] args) {
AtomicInteger atomicInteger = new AtomicInteger(2021);
//expect:期望 update：更新
// public final boolean compareAndSet(int expect, int update)
//如果我期望的值达到了，那么就更新，否则就不更新,CAS是CPU的并发原语
//====================捣乱的线程=========================
System.out.println(atomicInteger.compareAndSet(2021, 2022));
System.out.println(atomicInteger.get());
System.out.println(atomicInteger.compareAndSet(2022, 2021));
System.out.println(atomicInteger.get());
//===========================期望的线程=====================
System.out.println(atomicInteger.compareAndSet(2021, 6666));
System.out.println(atomicInteger.get());
}
}
20.原子引用
带版本号的原子操作！
解决ABA问题，引入原子引用！对应的思想是乐观锁
package com.cas;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicStampedReference;
public class CASDemo {
//CAS compareAndSet比较并交换
// AtomicStampedReference注意，如果泛型是一个包装类，注意对象的引用问题
//正常在业务操作，这里面比较的都是一个个对象
static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(1,1);
public static void main(String[] args) {
//AtomicInteger atomicInteger = new AtomicInteger(2021);
new Thread(()->{
int stamp = atomicStampedReference.getStamp();//获得版本号
System.out.println("a1=>"+stamp);
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
//拿到最新的版本号，拿到之后再把这个值加一
atomicStampedReference.compareAndSet(1, 2,
atomicStampedReference.getStamp(),
atomicStampedReference.getStamp() + 1);
System.out.println("a2=>"+atomicStampedReference.getStamp());
System.out.println(atomicStampedReference.compareAndSet(2, 1,
atomicStampedReference.getStamp(),
atomicStampedReference.getStamp() + 1));
//获取最新的版本号
System.out.println("a3=>"+atomicStampedReference.getStamp());
},"a").start();
new Thread(()->{
int stamp = atomicStampedReference.getStamp();//获得版本号
//乐观锁的原理相同
System.out.println("b1=>"+stamp);
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(atomicStampedReference.compareAndSet(1, 6, stamp, stamp + 1));
System.out.println("b2=>"+atomicStampedReference.getStamp());
},"b").start();
}
}
注意
Integer使用了对象缓存机制，默认范围是-128~127，推荐使用静态工厂方法valueOf获取对象实例，而不是new,因为valueOf使用缓存，而new 一定会创建新的对象分配新的内存空间；
21.各种锁的理解
1.公平锁，非公平锁
公平锁：非常公平，不能够插队，线程必须先来后到！
非公平锁：非常不公平，可以插队（默认都是非公平的）
public ReentrantLock() {
sync = new NonfairSync();
}
转换为非公平锁
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
2.可重入锁
可重入锁（递归锁）
Synchronized
package com.kuang.lock;
//Synchronized默认是非公平的
public class Demo01 {
public static void main(String[] args) {
Phone phone = new Phone();
new Thread(()->{
phone.sms();
},"A").start();
new Thread(()->{
phone.sms();
},"B").start();
}
}
class Phone{
//synchronized只有一把锁
public synchronized void sms(){
System.out.println(Thread.currentThread().getName() + "sms");
call();//这里也有锁
}
public synchronized void call(){
System.out.println(Thread.currentThread().getName() + "call");
}
}
Lock锁
package com.kuang.lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Demo02 {
public static void main(String[] args) {
Phone2 phone = new Phone2();
new Thread(()->{
phone.sms();
},"A").start();
new Thread(()->{
phone.sms();
},"B").start();
}
}
class Phone2{
Lock lock = new ReentrantLock();
public void sms(){
lock.lock();//细节问题：两把钥匙 第一把钥匙开外面的锁，另外一把开里面的锁
//lock.lock()负责解lock.unlock();
//lock 锁必须配对，否则就会死在里面
try {
System.out.println(Thread.currentThread().getName() + "sms");
call();//这里也有锁
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void call(){
lock.lock();
try {
System.out.println(Thread.currentThread().getName() + "call");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
3.自旋锁
spinlock
不断的尝试直到成功为止！
自定义锁来测试
package com.kuang.lock;
import java.util.concurrent.atomic.AtomicReference;
/**
* 自旋锁
*/
public class SpinlockDemo {
//int 类型 默认为0
//Thread引用类型 若为空认为null
AtomicReference<Thread> atomicReference = new AtomicReference();
//加锁
public void myLock(){
Thread thread = Thread.currentThread();
System.out.println(Thread.currentThread().getName() + "==> mylock");
//若为空 把线程丢进去 进行无限循环
//自旋锁
while (!atomicReference.compareAndSet(null,thread)){
}
}
//解锁
public void myUnLock(){
Thread thread = Thread.currentThread();
System.out.println(Thread.currentThread().getName() + "==> myUnlock");
//若为空 把线程丢进去 进行无限循环
//自旋锁
atomicReference.compareAndSet(thread,null);
}
}
测试
package com.kuang.lock;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;
public class TestSpinLock {
public static void main(String[] args) throws InterruptedException {
//ReentrantLock reentrantLock = new ReentrantLock();
// reentrantLock.lock();
// reentrantLock.unlock();
//底层使用的自旋锁CAS
SpinlockDemo lock = new SpinlockDemo();
new Thread(()->{
lock.myLock();
try {
TimeUnit.SECONDS.sleep(3);
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.myUnLock();
}
},"T1").start();
//延迟保证T1先获得锁
//T1解锁之后才会释放 T2才有机会进去拿到锁并且把它解锁掉
TimeUnit.SECONDS.sleep(1);
new Thread(()->{
lock.myLock();
try {
TimeUnit.SECONDS.sleep(3);
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.myUnLock();
}
},"T2").start();
}
}
结果：
Process finished with exit code 0
4.死锁
死锁测试，怎么排除死锁
死锁：
package com.kuang.lock;
import lombok.SneakyThrows;
import java.util.concurrent.TimeUnit;
public class DeadLockDemo {
public static void main(String[] args) {
String lockA = "lockA";
String lockB = "lockB";
new Thread(new MyThread(lockA,lockB),"T1").start();
new Thread(new MyThread(lockB,lockA),"T2").start();
}
}
class MyThread implements Runnable{
private String lockA;
private String lockB;
public MyThread(String lockA, String lockB) {
this.lockA = lockA;
this.lockB = lockB;
}
解决问题
1.使用jps -l定位进程号
2.使用jstack进程号查看进程信息（查看怎么死锁的）
面试或者工作中：排查问题：
1.日志
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