java并发中锁的原理之对象头

java对象头信息分析

从JVM源码中可知(markOop.hpp文件中)

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可以转换成这样的表格形式

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可以表示成下面这样
无锁： unused(25) hashcode(31) unused(1) age(4) biased_lock(1) lock(2)
偏向锁：thread(54) epoch(2) unused(1) age(4) biased_lock(1) lock(2)
轻量级锁： ptr_to_lock_record(62) lock(2)
重量级锁： ptr_to_heavyweight_monitor(62) lock(2)
Object Header 一共128btis(16字节)，klass部分可以压缩。压缩成12字节bytes
对象头包含两个部分-------mark word (64bits 8字节)和klass word(32bits 4字节)
可参考http://openjdk.java.net/groups/hotspot/docs/HotSpotGlossary.html

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mark word包括：hashcode哈希值、age表示gc年龄、blased_lock是否可偏向、lock锁的标识位
klass word：对象指针、填充区域

JOL来分析java的对象布局

加入依赖

<dependency>
       <groupId>org.openjdk.jol</groupId>
       <artifactId>jol-core</artifactId>
       <version>0.9</version>
</dependency>

对象头

示例代码

package com.thread;
public class A {
}

package com.thread;

import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;
public class MyJolTest1 {
    public static void main(String[] args) {
        System.out.println( VM.current().details());
        System.out.println( ClassLayout.parseClass(A.class).toPrintable());
    }
}

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Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]对应[Oop(Ordinary Object Pointer), boolean, byte, char,short, int, float, long, double]大小

boolean值在对象头展现

示例代码2

package com.thread;
public class A {
    boolean flag =false;
}

package com.thread;
import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;
public class MyJolTest2 {
    public static void main(String[] args) {
        System.out.println( VM.current().details());
        System.out.println( ClassLayout.parseClass(A.class).toPrintable());
    }
}

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16字节没有改变，其中对象头12字节

hashcode位置

001无锁

package com.thread;
import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;
public class MyJolTest3 {
    public static void main(String[] args) {
        A a= new A();
        System.out.println("befor hash");
        System.out.println(ClassLayout.parseInstance(a).toPrintable());
        //计算的hashcode
        System.out.println("-----------hashCode="+Integer.toHexString(a.hashCode()));
        System.out.println("after hash");
        System.out.println(ClassLayout.parseInstance(a).toPrintable());
    }
}

证明hashcode值是这31位

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偏向锁101

示例代码4

package com.thread;
import org.openjdk.jol.info.ClassLayout;
public class MyJolTest4 {
    static A a;
    public static void main(String[] args) throws Exception {
        Thread.sleep(5000);
        a = new A();
        System.out.println( "befre lock" );
        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
        sync();
        System.out.println( "after lock" );
        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
    }

    public static void sync() throws InterruptedException {
        synchronized (a) {
            System.out.println( "================" );
        }
    }
}

00000101 偏向
Thread.sleep(5000);代表可偏向，也可以修改启动参数XX:+UseBiasedLocking -XX:BiasedLockingStartupDelay=0

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轻量级锁000

示例代码5

package com.thread;
import org.openjdk.jol.info.ClassLayout;
public class MyJolTest5 {
    static A a;
    public static void main(String[] args) throws Exception {
        a = new A();
        System.out.println("befre lock");
        System.out.println(ClassLayout.parseInstance(a).toPrintable());
        sync();
        System.out.println("after lock");
        System.out.println(ClassLayout.parseInstance(a).toPrintable());
    }
    public static void sync() throws InterruptedException {
        synchronized (a){
            System.out.println("lock ing");
            System.out.println(ClassLayout.parseInstance(a).toPrintable());
        }
    }
}

从无锁到轻量级锁再到释放锁

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重量级锁010

示例代码

package com.thread;
import org.openjdk.jol.info.ClassLayout;
public class MyJolTest6 {
    static A a;
    public static void main(String[] args) throws Exception {
        a = new A();
        System.out.println( "befre lock" );
        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );

        Thread t1 = new Thread() {
            public void run() {
                synchronized (a) {
                    try {
                        Thread.sleep( 5000 );
                        System.out.println( "t1 release" );
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        t1.start();
        Thread.sleep( 1000 );
        System.out.println( "t1 lock ing" );
        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
        sync();

        System.gc();
        System.out.println( "after gc()" );
        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
    }

    public static void sync() throws InterruptedException {
        synchronized (a) {
            System.out.println( "t1 main lock" );
            System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
        }

    }
}

示例代码持有锁的变化状态无锁--轻量级锁--重量级锁--gc释放无锁

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wait方法调用，变成重量级锁

示例代码

package com.thread;
import org.openjdk.jol.info.ClassLayout;
public class MyJolTest7 {
    static A a;
    public static void main(String[] args) throws Exception {
        Thread.sleep(5000);
        a = new A();
        System.out.println("befre lock");
        System.out.println(ClassLayout.parseInstance(a).toPrintable());
        Thread t1= new Thread(){
            public void run() {
                synchronized (a) {
                    try {
                        System.out.println( "before wait" );
                        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
                        a.wait();
                        System.out.println( " after wait" );
                        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        t1.start();
        Thread.sleep(5000);
        synchronized (a) {
            a.notifyAll();
            }
        }
}

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计算hashcode之后，不可偏向

示例代码

package com.thread;
import org.openjdk.jol.info.ClassLayout;
public class MyJolTest8 {
    static A a;
    public static void main(String[] args) throws Exception {
        Thread.sleep( 5000 );
        a = new A();
        System.out.println( Integer.toHexString( a.hashCode() ) );
        System.out.println( "befre lock" );
        System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
        Thread t1 = new Thread() {
            public void run() {
                synchronized (a) {
                    System.out.println( "lock ed" );
                    System.out.println( ClassLayout.parseInstance( a ).toPrintable() );
                }
            }
        };
        t1.start();
    }
}

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总结：

主要展现java中的锁在对象头中的状态
偏向锁(101)、无锁(001)、轻量级锁(000)、重量级锁(010)、gc(011)
轻量级锁是交替持有锁(不存在竞争)，而重量级锁是互斥持有