30 minutes to thoroughly understand the synchronized lock upgrade process

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stay Java In the field of concurrent programming , We will use the lock during the meeting , For example, in the multi-threaded environment, in order to prevent some thread safety problems , There may be some unexpected problems , So I've sorted out a series about JDK The problem of middle lock , Help you understand them more deeply .

synchronized It's really a heavyweight lock ？

I believe most people have encountered this problem during the interview , The answer is No . This depends on JDK To judge . If JDK The version of is in 1.5 Before using synchronized The principle of lock is as follows ：

1. Add one before and after the resources that need to be locked “monitorenter” and “monitorexit” Instructions .

2. When a thread needs to enter this code critical area , Get involved first “ Grab the lock ”（ The essence is to get monitor Authority ）

3. If lock grabbing fails , Will be blocked , At this point, control can only be handed over to the operating system , It will start from  user mode  Switch to  kernel mode, The operating system is responsible for the scheduling between threads and the state change of threads , You need to switch between these two modes frequently （ Context conversion ）.

It can be seen that , Under the condition of the old mode, the cost of obtaining locks is relatively large , So later JDK The author of JDK Designed in Lock Interface , use CAS To implement the lock , To improve performance .

But when the competition is very fierce , use CAS The way may not get the lock all the time , No matter how much CAS It's also a waste CPU, The performance loss in this state will be higher than synchronized higher . So in this case , It's better to upgrade and lock directly , Let the operating system intervene .

Because of this , Therefore, there will be the saying of lock upgrade later .

synchronized Lock upgrade

Biased locking

stay synchronized In the process of upgrading , The first step will be upgraded to bias lock . The so-called deflection lock , Its essence is to let the lock remember the requested thread .

In most scenarios , In fact, there are more single thread access locks ,JDK The author of is reconstructing synchronized When , Designed a for the object header bit position , Specifically used to record lock information , Specifically, we can understand it through the following actual case ：

public static void main(String[] args) throws InterruptedException {
    Object o = new Object();
    System.out.println(" The synchronization block has not been entered ");
    System.out.println("markword:" + ClassLayout.parseInstance(o).toPrintable());
    // Default JVM There will be a warm-up phase during startup , Therefore, the deflection lock will not be opened by default 
    Thread.sleep(5000);
    Object b = new Object();
    System.out.println(" The synchronization block has not been entered ");
    System.out.println("markword:" + ClassLayout.parseInstance(b).toPrintable());
    synchronized (o){
        System.out.println(" Entering the synchronization block ");
        System.out.println("markword:" + ClassLayout.parseInstance(o).toPrintable());
    }
}

Pay attention to introducing some third-party dependencies , Help us view the information of the object header ：

<dependency>
    <groupId>org.openjdk.jol</groupId>
    <artifactId>jol-core</artifactId>
    // This version number is different , The format of the viewed content is also different 
    <version>0.16</version>
</dependency>

The console output is as follows ：

 The synchronization block has not been entered 
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
markword:java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 The synchronization block has not been entered 
markword:java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 Entering the synchronization block 
markword:java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x00007000050ee988 (thin lock: 0x00007000050ee988)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

In this case , If you carefully observe the contents of the console , You can find , When JVM When it started , The lock flag bit of the object head is unlocked . But after a whole child （ Probably 4 Seconds later ）, It will become a biasable The state of . If you need to adjust the delay time , You can use the parameter  -XX:BiasedLockingStartupDelay=0 To control .

Here I'll explain biasable The meaning of ：

biasable yes JVM Help us set the status , In this state , Once a thread accesses the lock , It's going to be direct CAS Modify the thread in the object header id. If it works , Then directly upgrade to bias lock . Otherwise, it will enter the next state of the lock -- Lightweight lock .

ps：JVM Because during the warm-up phase , There are many steps to use synchronized, So before just starting 4 In the second , Will not directly synchronized The mark of the lock is upgraded to biasable state . This is to reduce some unnecessary performance losses .

Lightweight lock

When a lock is accessed by a thread , It will become a biased lock state , When the new thread accesses the lock again , Will there be any change in the lock ？

Here I have sorted out a flow chart of lock changes , As shown below ：

To verify this process , We can practice through the following case ：

public static void main(String[] args) throws InterruptedException {
    //  sleep  5s
    Thread.sleep(5000);
    Object o = new Object();
    System.out.println(" Synchronization block not entered ,MarkWord  by ：");
    System.out.println(ClassLayout.parseInstance(o).toPrintable());
    synchronized (o){
        System.out.println((" Go into sync block ,MarkWord  by ："));
        System.out.println(ClassLayout.parseInstance(o).toPrintable());
    }
    Thread t2 = new Thread(() -> {
        synchronized (o) {
            System.out.println(" The new thread acquires the lock ,MarkWord by ：");
            System.out.println(ClassLayout.parseInstance(o).toPrintable());
        }
    });
    t2.start();
    t2.join();
    System.out.println(" The main thread looks at the lock object again ,MarkWord by ：");
    System.out.println(ClassLayout.parseInstance(o).toPrintable());
    synchronized (o){
        System.out.println((" The main thread enters the synchronization block again ,MarkWord  by ："));
        System.out.println(ClassLayout.parseInstance(o).toPrintable());
    }
    
    synchronized (b) {
        System.out.println((" The main thread enters the synchronization block again , And call hashcode Method ,MarkWord  by ："));
        b.hashCode();
        System.out.println(ClassLayout.parseInstance(b).toPrintable());
    }
}

Then let's observe the results of the implementation ：

/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/bin/java -agentlib:jdwp=transport=dt_socket,address=127.0.0.1:63267,suspend=y,server=n -javaagent:/Users/linhao/Library/Caches/IntelliJIdea2019.3/captureAgent/debugger-agent.jar -Dfile.encoding=UTF-8 -classpath "/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/tools.jar:/Users/linhao/IdeaProjects/my-github/concurrence-programming-lession/target/classes:/Users/linhao/.m2/repository/org/junit/jupiter/junit-jupiter/5.9.0-RC1/junit-jupiter-5.9.0-RC1.jar:/Users/linhao/.m2/repository/org/junit/jupiter/junit-jupiter-api/5.5.2/junit-jupiter-api-5.5.2.jar:/Users/linhao/.m2/repository/org/apiguardian/apiguardian-api/1.1.0/apiguardian-api-1.1.0.jar:/Users/linhao/.m2/repository/org/opentest4j/opentest4j/1.2.0/opentest4j-1.2.0.jar:/Users/linhao/.m2/repository/org/junit/platform/junit-platform-commons/1.5.2/junit-platform-commons-1.5.2.jar:/Users/linhao/.m2/repository/org/junit/jupiter/junit-jupiter-params/5.5.2/junit-jupiter-params-5.5.2.jar:/Users/linhao/.m2/repository/org/junit/jupiter/junit-jupiter-engine/5.5.2/junit-jupiter-engine-5.5.2.jar:/Users/linhao/.m2/repository/org/junit/platform/junit-platform-engine/1.5.2/junit-platform-engine-1.5.2.jar:/Users/linhao/.m2/repository/org/openjdk/jol/jol-core/0.16/jol-core-0.16.jar:/Users/linhao/.m2/repository/org/apache/dubbo/dubbo/2.7.8/dubbo-2.7.8.jar:/Users/linhao/.m2/repository/org/springframework/spring-context/5.2.5.RELEASE/spring-context-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/spring-aop/5.2.5.RELEASE/spring-aop-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/spring-beans/5.2.5.RELEASE/spring-beans-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/spring-expression/5.2.5.RELEASE/spring-expression-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/com/alibaba/spring/spring-context-support/1.0.8/spring-context-support-1.0.8.jar:/Users/linhao/.m2/repository/org/javassist/javassist/3.20.0-GA/javassist-3.20.0-GA.jar:/Users/linhao/.m2/repository/io/netty/netty-all/4.1.48.Final/netty-all-4.1.48.Final.jar:/Users/linhao/.m2/repository/com/google/code/gson/gson/2.8.6/gson-2.8.6.jar:/Users/linhao/.m2/repository/org/yaml/snakeyaml/1.25/snakeyaml-1.25.jar:/Users/linhao/.m2/repository/com/alibaba/fastjson/1.2.70/fastjson-1.2.70.jar:/Users/linhao/.m2/repository/org/apache/zookeeper/zookeeper/3.5.3-beta/zookeeper-3.5.3-beta.jar:/Users/linhao/.m2/repository/commons-cli/commons-cli/1.2/commons-cli-1.2.jar:/Users/linhao/.m2/repository/io/netty/netty/3.10.5.Final/netty-3.10.5.Final.jar:/Users/linhao/.m2/repository/org/apache/dubbo/dubbo-spring-boot-starter/2.7.8/dubbo-spring-boot-starter-2.7.8.jar:/Users/linhao/.m2/repository/org/apache/dubbo/dubbo-spring-boot-autoconfigure/2.7.8/dubbo-spring-boot-autoconfigure-2.7.8.jar:/Users/linhao/.m2/repository/org/apache/dubbo/dubbo-spring-boot-autoconfigure-compatible/2.7.8/dubbo-spring-boot-autoconfigure-compatible-2.7.8.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot-starter/2.2.6.RELEASE/spring-boot-starter-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot/2.2.6.RELEASE/spring-boot-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/2.2.6.RELEASE/spring-boot-autoconfigure-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot-starter-logging/2.2.6.RELEASE/spring-boot-starter-logging-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/Users/linhao/.m2/repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/Users/linhao/.m2/repository/org/slf4j/slf4j-api/1.7.30/slf4j-api-1.7.30.jar:/Users/linhao/.m2/repository/org/apache/logging/log4j/log4j-to-slf4j/2.12.1/log4j-to-slf4j-2.12.1.jar:/Users/linhao/.m2/repository/org/apache/logging/log4j/log4j-api/2.12.1/log4j-api-2.12.1.jar:/Users/linhao/.m2/repository/org/slf4j/jul-to-slf4j/1.7.30/jul-to-slf4j-1.7.30.jar:/Users/linhao/.m2/repository/jakarta/annotation/jakarta.annotation-api/1.3.5/jakarta.annotation-api-1.3.5.jar:/Users/linhao/.m2/repository/org/springframework/spring-core/5.2.5.RELEASE/spring-core-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/spring-jcl/5.2.5.RELEASE/spring-jcl-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot-starter-web/2.2.6.RELEASE/spring-boot-starter-web-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot-starter-json/2.2.6.RELEASE/spring-boot-starter-json-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.10.3/jackson-databind-2.10.3.jar:/Users/linhao/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.10.3/jackson-annotations-2.10.3.jar:/Users/linhao/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.10.3/jackson-core-2.10.3.jar:/Users/linhao/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jdk8/2.10.3/jackson-datatype-jdk8-2.10.3.jar:/Users/linhao/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jsr310/2.10.3/jackson-datatype-jsr310-2.10.3.jar:/Users/linhao/.m2/repository/com/fasterxml/jackson/module/jackson-module-parameter-names/2.10.3/jackson-module-parameter-names-2.10.3.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot-starter-validation/2.2.6.RELEASE/spring-boot-starter-validation-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/jakarta/validation/jakarta.validation-api/2.0.2/jakarta.validation-api-2.0.2.jar:/Users/linhao/.m2/repository/org/hibernate/validator/hibernate-validator/6.0.18.Final/hibernate-validator-6.0.18.Final.jar:/Users/linhao/.m2/repository/org/jboss/logging/jboss-logging/3.4.1.Final/jboss-logging-3.4.1.Final.jar:/Users/linhao/.m2/repository/com/fasterxml/classmate/1.5.1/classmate-1.5.1.jar:/Users/linhao/.m2/repository/org/springframework/spring-web/5.2.5.RELEASE/spring-web-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/spring-webmvc/5.2.5.RELEASE/spring-webmvc-5.2.5.RELEASE.jar:/Users/linhao/.m2/repository/org/springframework/boot/spring-boot-starter-tomcat/2.2.6.RELEASE/spring-boot-starter-tomcat-2.2.6.RELEASE.jar:/Users/linhao/.m2/repository/org/apache/tomcat/embed/tomcat-embed-core/9.0.33/tomcat-embed-core-9.0.33.jar:/Users/linhao/.m2/repository/org/apache/tomcat/embed/tomcat-embed-el/9.0.33/tomcat-embed-el-9.0.33.jar:/Users/linhao/.m2/repository/org/apache/tomcat/embed/tomcat-embed-websocket/9.0.33/tomcat-embed-websocket-9.0.33.jar:/Users/linhao/.m2/repository/org/apache/curator/curator-framework/2.12.0/curator-framework-2.12.0.jar:/Users/linhao/.m2/repository/org/apache/curator/curator-client/2.12.0/curator-client-2.12.0.jar:/Users/linhao/.m2/repository/com/google/guava/guava/16.0.1/guava-16.0.1.jar:/Users/linhao/.m2/repository/org/apache/curator/curator-recipes/2.12.0/curator-recipes-2.12.0.jar:/Users/linhao/.m2/repository/com/alibaba/transmittable-thread-local/2.12.2/transmittable-thread-local-2.12.2.jar:/Users/linhao/.m2/repository/org/apache/rocketmq/rocketmq-client/4.8.0/rocketmq-client-4.8.0.jar:/Users/linhao/.m2/repository/org/apache/rocketmq/rocketmq-common/4.8.0/rocketmq-common-4.8.0.jar:/Users/linhao/.m2/repository/org/apache/rocketmq/rocketmq-remoting/4.8.0/rocketmq-remoting-4.8.0.jar:/Users/linhao/.m2/repository/org/apache/rocketmq/rocketmq-logging/4.8.0/rocketmq-logging-4.8.0.jar:/Users/linhao/.m2/repository/io/netty/netty-tcnative-boringssl-static/2.0.30.Final/netty-tcnative-boringssl-static-2.0.30.Final.jar:/Users/linhao/.m2/repository/commons-validator/commons-validator/1.6/commons-validator-1.6.jar:/Users/linhao/.m2/repository/commons-beanutils/commons-beanutils/1.9.2/commons-beanutils-1.9.2.jar:/Users/linhao/.m2/repository/commons-digester/commons-digester/1.8.1/commons-digester-1.8.1.jar:/Users/linhao/.m2/repository/commons-logging/commons-logging/1.2/commons-logging-1.2.jar:/Users/linhao/.m2/repository/commons-collections/commons-collections/3.2.2/commons-collections-3.2.2.jar:/Users/linhao/.m2/repository/org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar"  Concurrent programming 03. View memory layout information .MarkWordDemo_4
Connected to the target VM, address: '127.0.0.1:63267', transport: 'socket'
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
 Synchronization block not entered ,MarkWord  by ：java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 Synchronization block not entered ,MarkWord  by ：java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 Go into sync block ,MarkWord  by ：java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x00007fe8a5009805 (biased: 0x0000001ffa294026; epoch: 0; age: 0)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 The new thread acquires the lock ,MarkWord by ：
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000070000ba03908 (thin lock: 0x000070000ba03908)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 The main thread looks at the lock object again ,MarkWord by ：
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 The main thread enters the synchronization block again ,MarkWord  by ：
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000700009f87980 (thin lock: 0x0000700009f87980)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


 The main thread enters the synchronization block again , And call hashcode Method ,MarkWord  by ：
java.lang.Object object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x00007fe8a51391ea (fat lock: 0x00007fe8a51391ea)
  8   4        (object header: class)    0xf80001e5
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total


Disconnected from the target VM, address: '127.0.0.1:63267', transport: 'socket'


Process finished with exit code 0

Through the print content in the console, we can find , The state of the lock has gone through the following steps ：

• biasable state

In this state , Lock is a state to be biased , At this time, if there is a thread request . But if it is just started JVM The state of the word , The head of the object will be non-biasable state , Only wait jvm Warm up for a period of time （ It's about 4 second ）, Will stay and turn into biasable state .

• biased state

When the first request obtains the lock , The lock state will become biased to the lock state , That is to say biased. If you are in the biased lock state , There are also new threads involved in lock grabbing , Then lock escalation will occur , Enter the lightweight lock state stage .

• thin lock state

You can see , When a lock has experienced a biased lock state , Later, if another thread accesses it , It will be upgraded to the state of lightweight lock , That is to say thin lock state .

• fat lock state

When we call hashcode Method time , Will find , The head of the lock object will have an extra one called fat lock The state of , That means , At this time, the lock has been upgraded to a heavyweight lock .

Heavyweight lock

When a lock is already in the state of lightweight lock , If there are more threads trying to acquire the lock , Then the lock will be accessed by multiple threads in a spinning way , When the number of visits reaches a certain limit ,synchronized It will be automatically upgraded to the state of heavyweight lock .

When upgrading to a heavyweight lock , Lock object's mark word The pointer in no longer points to lock record, It points to the lock object in the heap monitor object . When multiple threads access synchronous code at the same time , These threads will first try to get the corresponding value of the current lock object monitor The ownership of the ：

• To be successful , Determine whether the current thread is reentrant , If it's reentry, then recursions+1

• Acquisition failure , The current thread will be blocked , Waiting for other threads to be awakened after unlocking , Compete for lock objects again

In the case of heavyweight locks , The process of adding and unlocking involves the operation of the operating system Mutex Lock Do mutually exclusive operations , The scheduling between threads and the state change process of threads need to switch between user state and core state , It's going to consume a lot of cpu resources , Resulting in reduced performance .

What are several ways to upgrade a lock to a heavyweight state ？

• call wait Method

• Call the... Of the object in the synchronized code block hashcode Method

Finally, I drew a flow chart of lock upgrade to share with you ：

Summary

Actually JVM Have been to synchronized optimized . It can be used directly , As for the strength of the lock ,JVM The bottom layer is ready. We can use it directly . But as an engineer , Understanding these underlying principles can still increase our internal skills .

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