多线程下的Volatile介绍

volatile

特性

• 可见性

可见性: 当一个线程更新主内存的变量值,即刻通知其他线程重新读取该内存变量值

• 不保证原子性

原子性: 操作不可分割,当一个线程操作某个业务时候,中间不可进行加塞,保证这个过程是完整的,要么同时成功要么同时失败.

• 禁止指令重排

指令重排: JVM在编译Java代码的时候，或者CPU在执行JVM字节码的时候，对现有的指令顺序进行重新排序.为了在执行程序时提高性能,编辑器和处理器常常会对指令做重排

例子(代码说明)

• 可见性

class MyData {
    volatile int number = 0;
    public void AddNumber() {
        this.number = 60;
    }
}
public class MainBases01 {
    public static void main(String[] args) throws InterruptedException {
        MyData myData = new MyData();
//        第一个线程
        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + "\t come in!  myData.number=" + myData.number);
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            myData.AddNumber();
            System.out.println(Thread.currentThread().getName() + "\t AddNumber()!  myData.number=" + myData.number);
        }, "AAA").start();

        //验证可见性方法一:可以另起一个线程  可以感知到第一个线程感知到被volatile修饰的number值
        new Thread(()->{
            System.out.println(Thread.currentThread().getName()+"\t come in!  myData.number="+myData.number);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName()+"\t myData.AddNumber()!  myData.number="+myData.number);
        },"BBB").start();
//        验证可见性方法二:在main线程进行验证,没有加volatile修饰的话下面这个会一直循环.
//        while (myData.number == 0) {
//        }
        System.out.println(Thread.currentThread().getName() + "\t myData.number=" + myData.number);
    }
}

• 不保证原子性

class MyData {
    volatile int number = 0;
    AtomicInteger atomicInteger=new AtomicInteger();
    public void AddNumber() {
        this.number = 60;
    }
    public void AddPlus() {
        this.number++;
    }
    public void AddAtomicInteger(){
        atomicInteger.getAndIncrement();
    }
}
public class MainBases01 {
    public static void main(String[] args) throws InterruptedException {
        MyData myData = new MyData();
        for (int i = 0; i < 20; i++) {
            new Thread(()->{
                for (int j = 0; j < 1000; j++) {
                    myData.AddPlus();
                    myData.AddAtomicInteger();
                }
            },String.valueOf(i)).start();
        }
        while (Thread.activeCount()>2){//一个main线程,另一个是GC线程
            Thread.yield();
        }
        System.out.println("验证volatile的原子性"+myData.number);
    }
}

不保证原子性的解决办法

class MyData {
    AtomicInteger atomicInteger=new AtomicInteger();//定义AtomicInteger 该类保证原子性
    public void AddAtomicInteger(){
        atomicInteger.getAndIncrement();
    }
}

• AtomicInteger的源码剖析

原理也很简单,就是使用了unsafe类getAndAddInt()方法

原理就是CAS方法: 利用native修饰的compareAndSwapInt()(也被叫做:本地方法,这个方法是可以被C语言重写).

理论过程原理: 就是一直循环进行变量比较并交换原则:比较主内存的值是否和期望值一致,一致的话就进行交换主内存的值.[CAS下次再细讲OK?]

//
public final class Unsafe {
    public final int getAndAddInt(Object var1, long var2, int var4) {
        int var5;
        do {
            var5 = this.getIntVolatile(var1, var2);
        } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
        return var5;
    }
    //定义的compareAndSwapInt()方法是native修饰
    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);
}

禁止指令重排

指令重排的概念

    1--->  int a = 2  << 1;
    2--->  int b = 3 << 1;
    3--->  int result = a * b + 2333;

相信你看到这段简短的代码，也会知道这段代码的执行顺序，1 - > 2 -> 3,相信你会觉得这就是代码执行的顺序，但是在jvm中是不会这样执行的，这里我们可以用着三个变量的依赖关系来解释一下原因

这三个变量中 1 与 2没有依赖关系，3与 1和2 都有依赖关系，也就是说，没有依赖关系的两段代码即使我们将他们编译执行的顺序进行调换，这样也不会对代码的结果产生改变 也就是 上面的代码在jvm中实际上是 2 -> 1 -> 3

当然在单线程中JVM对代码进行指令重排并不会产生影响，但是在多线程中进行指令重排的话就会产生一些不确定的结果了，现在我们来看一下指令重排在多线程的一个经典的例子，单例模式，懒加载

public class Test {
    private static Test instance = null;

    public static Test getSingInstance() {
        if (instance == null) {
            synchronized (Test.class) {
                instance = new Test();
            }
        }
        return  instance;
    }
    public void action() {
        System.out.println(">>>>>>>>>>");
    }

    public static void main(String[] args) {

    }
}

• 可以自己反编译进行查看(如上的类反编译如下)

Classfile /D:/demo/target/classes/com/example/demo/Volatile/Test.class
  Last modified 2022-5-15; size 978 bytes
  MD5 checksum f0d641de5be26149e162978843fd9c90
  Compiled from "Test.java"
public class com.example.demo.Volatile.Test
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#31         // java/lang/Object."<init>":()V
   #2 = Fieldref           #3.#32         // com/example/demo/Volatile/Test.instance:Lcom/example/demo/Volatile/Test;
   #3 = Class              #33            // com/example/demo/Volatile/Test
   #4 = Methodref          #3.#31         // com/example/demo/Volatile/Test."<init>":()V
   #5 = Fieldref           #34.#35        // java/lang/System.out:Ljava/io/PrintStream;
   #6 = String             #36            // >>>>>>>>>>
   #7 = Methodref          #37.#38        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #8 = Class              #39            // java/lang/Object
   #9 = Utf8               instance
  #10 = Utf8               Lcom/example/demo/Volatile/Test;
  #11 = Utf8               <init>
  #12 = Utf8               ()V
  #13 = Utf8               Code
  #14 = Utf8               LineNumberTable
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               this
  #17 = Utf8               getSingInstance
  #18 = Utf8               ()Lcom/example/demo/Volatile/Test;
  #19 = Utf8               StackMapTable
  #20 = Class              #39            // java/lang/Object
  #21 = Class              #40            // java/lang/Throwable
  #22 = Utf8               action
  #23 = Utf8               main
  #24 = Utf8               ([Ljava/lang/String;)V
  #25 = Utf8               args
  #26 = Utf8               [Ljava/lang/String;
  #27 = Utf8               MethodParameters
  #28 = Utf8               <clinit>
  #29 = Utf8               SourceFile
  #30 = Utf8               Test.java
  #31 = NameAndType        #11:#12        // "<init>":()V
  #32 = NameAndType        #9:#10         // instance:Lcom/example/demo/Volatile/Test;
  #33 = Utf8               com/example/demo/Volatile/Test
  #34 = Class              #41            // java/lang/System
  #35 = NameAndType        #42:#43        // out:Ljava/io/PrintStream;
  #36 = Utf8               >>>>>>>>>>
  #37 = Class              #44            // java/io/PrintStream
  #38 = NameAndType        #45:#46        // println:(Ljava/lang/String;)V
  #39 = Utf8               java/lang/Object
  #40 = Utf8               java/lang/Throwable
  #41 = Utf8               java/lang/System
  #42 = Utf8               out
  #43 = Utf8               Ljava/io/PrintStream;
  #44 = Utf8               java/io/PrintStream
  #45 = Utf8               println
  #46 = Utf8               (Ljava/lang/String;)V
{
  public com.example.demo.Volatile.Test();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/example/demo/Volatile/Test;

  public static com.example.demo.Volatile.Test getSingInstance();
    descriptor: ()Lcom/example/demo/Volatile/Test;
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=0
         0: getstatic     #2                  // Field instance:Lcom/example/demo/Volatile/Test;
         3: ifnonnull     31
         6: ldc           #3                  // class com/example/demo/Volatile/Test
         8: dup
         9: astore_0
        10: monitorenter
        11: new           #3                  // class com/example/demo/Volatile/Test
        14: dup
        15: invokespecial #4                  // Method "<init>":()V
        18: putstatic     #2                  // Field instance:Lcom/example/demo/Volatile/Test;
        21: aload_0
        22: monitorexit
        23: goto          31
        26: astore_1
        27: aload_0
        28: monitorexit
        29: aload_1
        30: athrow
        31: getstatic     #2                  // Field instance:Lcom/example/demo/Volatile/Test;
        34: areturn
      Exception table:
         from    to  target type
            11    23    26   any
            26    29    26   any
      LineNumberTable:
        line 7: 0
        line 8: 6
        line 9: 11
        line 10: 21
        line 12: 31
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 26
          locals = [ class java/lang/Object ]
          stack = [ class java/lang/Throwable ]
        frame_type = 250 /* chop */
          offset_delta = 4

  public void action();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #6                  // String >>>>>>>>>>
         5: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 15: 0
        line 16: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  this   Lcom/example/demo/Volatile/Test;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 20: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0  args   [Ljava/lang/String;
    MethodParameters:
      Name                           Flags
      args

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: aconst_null
         1: putstatic     #2                  // Field instance:Lcom/example/demo/Volatile/Test;
         4: return
      LineNumberTable:
        line 4: 0
}
SourceFile: "Test.java"

看不懂的可以查阅相关字典(字节码字典字典如下)

看似一句简单的instance = new Test(); 实际分三步

• memory =allocate(); //1：分配对象的内存空间
• ctorInstance(memory); //2：初始化对象
• instance =memory; //3：设置instance指向刚分配的内存地址

可以看出第一条指令和第二条指令有依赖关系, 但第三条指令和另外两条指令就没有依赖关系,这时候JVM指令重排1->3->2
那么问题来了, 执行1后就分配了内存空间,再执行3判断instance!=null.但是还没完成初始化就会运行报错了!所以加上volatile来禁止指令重排.