语言基础 / Java15 分钟 读完 (大约 2311 个字)

分析 Synchronized 锁的4个状态

1.前言

通过JOL查看对象的头信息，Synchronized 关键字锁的状态：无锁、偏向锁、轻量级锁、重量级锁

  • 基于64位 jdk8测试

int pthread_mutex_lock(pthread_mutex_t *mutex);

pthread_mutex_lock()返回时，该互斥锁已被锁定。线程调用该函数让互斥锁上锁，如果该互斥锁已被另一个线程锁定和拥有，则调用该线程将阻塞，直到该互斥锁变为可用为止。 对于 Solaris线程，请参见mutex_lock 语法

  • 会阻塞


我们可以从JVM 源码中看到对glibc库调用，操作系统层级的锁

ThreadCritical::ThreadCritical

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ThreadCritical::ThreadCritical() {
  pthread_t self = pthread_self();
  if (self != tc_owner) {
    int ret = pthread_mutex_lock(&tc_mutex);
    guarantee(ret == 0, "fatal error with pthread_mutex_lock()");
    assert(tc_count == 0, "Lock acquired with illegal reentry count.");
    tc_owner = self;
  }
  tc_count++;
}

这个是底层系统库，我们是看不到源码的，只能自己下载。可以在glibc/nptl/phread_mutex_lock.c里看到实现。

2.验证准备

  • 先通过一个jni调用来获取pthread层面的tid

    JniTool.java

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    package jnitool;

    public class JniTool {
        static {
            System.loadLibrary("jnitools");
        }
        public native void getThreadID();
    }
    这里jnitool必须在 java.library.path这一jvm变量所指向的路径中，
    可以通过如下方法获得该变量System.getProperty(“java.library.path”);

生成.h文件，javah -jni jnitool.JniTool

打印java线程的pid和tid，jvm层面

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#include "jnitool_JniTool.h"
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

JNIEXPORT void JNICALL Java_jnitool_JniTool_getThreadID
  (JNIEnv * env, jobject obj){
        fprintf(stdout,"java_lock log  PID = %d TID = %lu \n",getpid(),pthread_self());
        fflush(stdout);//刷新，防止错位打印
}

使用make编译

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vim makefile

libjnitools.so:
	gcc -lpthread -fPIC -shared -o libjnitools.so  -I /root/jdk1.8.0_221/include/ -I /root/jdk1.8.0_221/include/linux/ ./jnitools.c
  
make

对象头信息 object header 关闭指针压缩 -XX:-UseCompressedOops

  • 关于对象头信息的详情介绍，可以

  • 自己写个工具方法，根据markworld转换出头部信息

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    |--------------------------------------------------------------------------------------------------------------|
    |                                              Object Header (128 bits)                                        |
    |--------------------------------------------------------------------------------------------------------------|
    |                        Mark Word (64 bits)                                    |      Klass Word (64 bits)    |       
    |--------------------------------------------------------------------------------------------------------------|
    |  unused:25 | identity_hashcode:31 | unused:1 | age:4 | biased_lock:1 | lock:2 |     OOP to metadata object   |  无锁
    |----------------------------------------------------------------------|--------|------------------------------|
    |  thread:54 |         epoch:2      | unused:1 | age:4 | biased_lock:1 | lock:2 |     OOP to metadata object   |  偏向锁
    |----------------------------------------------------------------------|--------|------------------------------|
    |                     ptr_to_lock_record:62                            | lock:2 |     OOP to metadata object   |  轻量锁
    |----------------------------------------------------------------------|--------|------------------------------|
    |                     ptr_to_heavyweight_monitor:62                    | lock:2 |     OOP to metadata object   |  重量锁
    |----------------------------------------------------------------------|--------|------------------------------|
    |                                                                      | lock:2 |     OOP to metadata object   |    GC
    |--------------------------------------------------------------------------------------------------------------|

  • 接受JOL输出，自己写一个转换工具，方便debug

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    public void parseHeaderInfo(String printable) {
            String[] split = printable.replaceAll("[|)]", "").split("\n");
            StringBuilder builder = new StringBuilder();
            for (int i = 2; i <= 3; i++) {
                builder.append(split[i].split("\\(")[2]);
            }
            ArrayList<String> strings = new ArrayList<>(Arrays.asList(builder.toString().split(" ")));
            Collections.reverse(strings);
            builder = new StringBuilder();
            for (String string : strings) {
                builder.append(string);
            }
            String markWord = builder.toString();
            System.out.println("\n--------START-----------");
            //System.out.println("mark world:"+markWord);
            String lock = markWord.substring(62);
            String biased_lock = String.valueOf(markWord.charAt(61));
            builder = new StringBuilder();
            if (lock.equals("01")) {
                if (biased_lock.equals("0")) {
                    System.out.println("--------无锁-----------");
                    System.out.println("unused:25:" + markWord.substring(0, 25));
                    System.out.println("identity_hashcode:31: " + markWord.substring(25, 56)
                            + " (Hex:" + Integer.toHexString(Integer.parseInt(markWord.substring(25, 56), 2)) + ")");
                    System.out.println("unused:1:" + markWord.substring(56, 57));
                    System.out.println("age:4:" + markWord.substring(57, 61));
                    System.out.println("biased_lock:1:" + markWord.substring(61, 62));
                    System.out.println("lock:2:" + markWord.substring(62));

                } else {
                    System.out.println("--------偏向锁--------");
                    System.out.println("thread:54:" + markWord.substring(0, 54)
                            +"(Dex:"+Long.parseLong(markWord.substring(0, 54), 2)+")");
                    System.out.println("epoch:2:" + markWord.substring(54, 56));
                    System.out.println("unused:1:" + markWord.substring(56, 57));
                    System.out.println("age:4:" + markWord.substring(57, 61));
                    System.out.println("biased_lock:1:" + markWord.substring(61, 62));
                }
            } else {
                String hexString = Long.toHexString(Long.parseLong(markWord.substring(0, 62), 2));
                if (lock.equals("00")) {
                    System.out.println("--------轻量级锁--------");
                    System.out.println("ptr_to_lock_record:62:" + markWord.substring(0, 62)
                            + " (Hex:" + hexString + ")");
                    System.out.println("lock:2:" + markWord.substring(62));
                } else if (lock.equals("10")) {
                    System.out.println("--------重量级锁--------");
                    System.out.println("ptr_to_heavyweight_monitor::62:" + markWord.substring(0, 62)
                            + " (Hex:" + hexString + ")");
                    System.out.println("lock:2:" + markWord.substring(62));
                } else if (lock.equals("11")) {
                    System.out.println("--------GC标记--------");
                }
            }
            System.out.println("--------END-----------\n");
        }

3.无锁

  • JVM直接优化，进行无锁访问，虽然加了锁

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    import jnitools.JniTools;
    import org.openjdk.jol.info.ClassLayout;

    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.Collections;

    public class SyncTest {
        final Object lock = new Object();
        JniTools jniTool = new JniTools();

        public static void main(String[] args) throws InterruptedException {
            SyncTest test = new SyncTest();
            test.parseHeaderInfo((ClassLayout.parseInstance(test.lock).toPrintable()));
            System.out.println("---------------------------start--------------------");
            test.start();
        }

        public void start() throws InterruptedException {
            Runnable runnable = new Runnable() {
                int count = 0;

                @Override
                public void run() {
                    while (true) {
                        if (count < 100) {
                            count++;
                        } else {
                            return;
                        }
                        try {
                            lock();
                            parseHeaderInfo(ClassLayout.parseInstance(lock).toPrintable());
                            Thread.sleep(200);
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                }
            };
            new Thread(runnable).start();
            Thread.sleep(5000);
            new Thread(runnable).start();
        }

        public void lock() {
            synchronized (lock) {
                jniTool.getTid();
            }
        }
    }

  • 分析，虽然有多个线程访问，但是不存在，同时竞争。


    log 无锁
    out.log

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    --------START-----------
    --------无锁-----------
    unused:25:0000000000000000000000000
    identity_hashcode:31: 0000000000000000000000000000000 (Hex:0)
    unused:1:0
    age:4:0000
    biased_lock:1:0
    lock:2:01
    --------END-----------

    ---------------------------start--------------------
    java_lock log-----------PID = 189 TID = 139775743776512 

    --------START-----------
    --------无锁-----------
    unused:25:0000000000000000000000000
    identity_hashcode:31: 0000000000000000000000000000000 (Hex:0)
    unused:1:0
    age:4:0000
    biased_lock:1:0
    lock:2:01
    --------END-----------

    java_lock log-----------PID = 189 TID = 139775743776512 

    --------START-----------
    --------无锁-----------
    unused:25:0000000000000000000000000
    identity_hashcode:31: 0000000000000000000000000000000 (Hex:0)
    unused:1:0
    age:4:0000
    biased_lock:1:0
    lock:2:01
    --------END-----------

    java_lock log-----------PID = 189 TID = 139775743776512 

    --------START-----------
    --------无锁-----------
    unused:25:0000000000000000000000000
    identity_hashcode:31: 0000000000000000000000000000000 (Hex:0)
    unused:1:0
    age:4:0001
    biased_lock:1:0
    lock:2:01
    --------END-----------

4 偏向锁

  • 当关闭偏向锁功能时
  • 由于多个线程竞争偏向锁导致偏向锁升级为轻量级锁。
  • 一开始就是偏向锁，处于可偏向，谁先来锁，就偏向谁
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    import jnitools.JniTools;
    import org.openjdk.jol.info.ClassLayout;

    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.Collections;

    public class SyncTest {
        final Object lock = new Object();
        JniTools jniTool = new JniTools();

        public static void main(String[] args) throws InterruptedException {
            Thread.sleep(5000);
            SyncTest test = new SyncTest();
            test.parseHeaderInfo((ClassLayout.parseInstance(test.lock).toPrintable()));
            System.out.println("---------------------------start--------------------");
            test.start();
            Thread.sleep(50000);
        }

        public void start() throws InterruptedException {
            Runnable runnable = new Runnable() {
                @Override
                public void run() {
                    int count = 0;
                    while (true) {
                        if (count < 50) {
                            count++;
                        } else {
                            System.out.println("Thread "+Thread.currentThread().getName()+" return ");
                            return;
                        }
                        try {
                            lock();
                            parseHeaderInfo(ClassLayout.parseInstance(lock).toPrintable());
                            // Thread.sleep(100);
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                }
            };
            new Thread(runnable).start();
            Thread.sleep(5000);
            new Thread(runnable).start();
        }

        public void lock() {
            synchronized (lock) {
                jniTool.getTid();
            }
        }
    }

  • Thread.sleep(5000); 主要在线程启动时候加上了sleep

    out.log

  • 为了减少JVM启动时初始化时间，JVM默认延时加载偏向锁。大概为4s左右，具体时间因机器而异。当然我们也可以设置JVM参数 -XX:BiasedLockingStartupDelay=0 来取消延时加载偏向锁

  • 你会发现占用 thread 和 epoch 的 位置的均为0，说明当前偏向锁并没有偏向任何线程，此时这个偏向锁正处于可偏向状态。

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    --------START-----------
    --------偏向锁--------
    thread:54:000000000000000000000000000000000000000000000000000000(Dex:0)
    epoch:2:00
    unused:1:0
    age:4:0000
    biased_lock:1:1
    --------END-----------

    ---------------------------start--------------------
    java_lock log-----------PID = 398 TID = 140630884325120 

    --------START-----------
    --------偏向锁--------
    thread:54:000000000000000001111111111001110100010000100001101100(Dex:137335212140)
    epoch:2:00
    unused:1:0
    age:4:0001
    biased_lock:1:1
    --------END-----------

    java_lock log-----------PID = 398 TID = 140630884325120 

    --------START-----------
    --------偏向锁--------
    thread:54:000000000000000001111111111001110100010000100001101100(Dex:137335212140)
    epoch:2:00
    unused:1:0
    age:4:0001
    biased_lock:1:1
    --------END-----------

  • Thread Thread-1 return 当线程2开始的时候，线程1已结结束了，这里怀疑线程复用，因为这里不满足线程重新偏向，且线程没有重新偏向

  • 偏向的线程没有变化，且pthread的TID的对象头的里的信息都没有变化

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    java_lock log-----------PID = 398 TID = 140630884325120 

    --------START-----------
    --------偏向锁--------
    thread:54:000000000000000001111111111001110100010000100001101100(Dex:137335212140)
    epoch:2:00
    unused:1:0
    age:4:0001
    biased_lock:1:1
    --------END-----------

    Thread Thread-1 return 
    java_lock log-----------PID = 398 TID = 140630884325120 

    --------START-----------
    --------偏向锁--------
    thread:54:000000000000000001111111111001110100010000100001101100(Dex:137335212140)
    epoch:2:00
    unused:1:0
    age:4:0001
    biased_lock:1:1
    --------END-----------

5 轻量级锁

  • 我们让线程1放锁后，不让线程1退出

  • 让线程2去拿锁

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      import jnitool.JniTool;
    import org.openjdk.jol.info.ClassLayout;

    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.Collections;

    public class SyncTest {
        final Object lock = new Object();
        JniTool jniTool = new JniTool();

        public static void main(String[] args) throws InterruptedException {
            Thread.sleep(5000);
            SyncTest test = new SyncTest();
            test.parseHeaderInfo((ClassLayout.parseInstance(test.lock).toPrintable()));
            System.out.println("---------------------------start--------------------");
            test.start();
        }

        public void start() throws InterruptedException {
            Runnable runnable = new Runnable() {
                @Override
                public void run() {
                    int count = 0;
                    System.out.println(Thread.currentThread().getName() + " t1 start");
                    while (true) {
                        if (count < 50) {
                            count++;
                        } else {
                         return;
                        }
                        lock();
                    }
                }
            };
            new Thread(runnable).start();
            Thread.sleep(1000);//这里保证退出
            new Thread(runnable).start();
        }
        public void lock() {
            synchronized (lock) {
                Class<?> aClass = lock.getClass();
                jniTool.getThreadID();
                parseHeaderInfo(ClassLayout.parseInstance(lock).toPrintable());
            }
        }
    }

  • 线程1执行的时候，是偏向了线程1的

  • 线程1退出，线程2来的时候，直接升级了轻量级锁

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    java_lock log  PID = 61919   TID = 123145379807232 

    --------START-----------Thread-1
    --------偏向锁--------
    thread:54:000000000000000001111111110000101010110100010011100100(Dex:137181742308)
    epoch:2:00
    unused:1:0
    age:4:0000
    biased_lock:1:1
    --------END-----------Thread-1

    Thread-2 t1 start
    java_lock log  PID = 61919   TID = 123145379807232 

    --------START-----------Thread-2
    --------轻量级锁--------
    ptr_to_lock_record:62:00000000000000000111000000000000000001001001111001111000001110 (Hex:1c0001279e0e)
    lock:2:00
    --------END-----------Thread-2

    java_lock log  PID = 61919   TID = 123145379807232

6. 重量级锁

  • 偏向锁直接升级

  • 2个线程，同时并发访问

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    import jnitool.JniTool;
    import org.openjdk.jol.info.ClassLayout;

    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.Collections;

    public class SyncTest {
        final Object lock = new Object();
        JniTool jniTool = new JniTool();

        public static void main(String[] args) throws InterruptedException {
            Thread.sleep(5000);
            SyncTest test = new SyncTest();
            test.parseHeaderInfo((ClassLayout.parseInstance(test.lock).toPrintable()));
            System.out.println("---------------------------start--------------------");
            test.start();
        }

        public void start() throws InterruptedException {
            Runnable runnable = new Runnable() {
                @Override
                public void run() {
                    int count = 0;
                    System.out.println(Thread.currentThread().getName() + " t1 start");
                    while (true) {
                        if (count < 50) {
                            count++;
                        } else {
                         return;
                        }
                        lock();
                    }
                }
            };
            new Thread(runnable).start();
            new Thread(runnable).start();
        }
        public void lock() {
            synchronized (lock) {
                Class<?> aClass = lock.getClass();
                jniTool.getThreadID();
                parseHeaderInfo(ClassLayout.parseInstance(lock).toPrintable());
            }
        }
    }

  • 从可偏向状态，直接升级为重量级锁。

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    --------START-----------main
    --------偏向锁--------
    thread:54:000000000000000000000000000000000000000000000000000000(Dex:0)
    epoch:2:00
    unused:1:0
    age:4:0000
    biased_lock:1:1
    --------END-----------main

    ---------------------------start--------------------
    Thread-1 t1 start
    Thread-2 t1 start
    java_lock log  PID = 61884   TID = 123145554096128 

    --------START-----------Thread-1
    --------重量级锁--------
    ptr_to_heavyweight_monitor::62:00000000000000000111111110101101000111010000000110001011010010 (Hex:1feb474062d2)
    lock:2:10
    --------END-----------Thread-1

总结

  • 初始状态可能是偏向锁的可偏向状态，也可能是无锁状态
  • 初始锁，若只有一个线程连续的多次访问，构成锁的偏向
  • 不构成锁的重新偏向情况下，后面有多个线程轮流访问，不构成锁的竞争，锁升级到轻量级锁
  • 可以有连续状态，连续锁的升级。
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