30分钟彻底弄懂 synchronized 锁升级过程( 二 ) _synchronized

轻量级锁当锁被一个线程访问的时候，它会变成偏向锁的状态，那么当新的线程再次访问该锁的时候，锁会有什么变化吗？
这里我整理了一张锁的变化流程图，如下所示：

文章插图

为了验证这个过程，我们可以通过下边这个案例来实践下：

public static void main(String[] args) throws InterruptedException {// 睡眠 5sThread.sleep(5000);Object o = new Object();System.out.println("未进入同步块，MarkWord 为：");System.out.println(ClassLayout.parseInstance(o).toPrintable());synchronized (o){System.out.println(("进入同步块，MarkWord 为："));System.out.println(ClassLayout.parseInstance(o).toPrintable());}Thread t2 = new Thread(() -> {synchronized (o) {System.out.println("新线程获取锁，MarkWord为：");System.out.println(ClassLayout.parseInstance(o).toPrintable());}});t2.start();t2.join();System.out.println("主线程再次查看锁对象，MarkWord为：");System.out.println(ClassLayout.parseInstance(o).toPrintable());synchronized (o){System.out.println(("主线程再次进入同步块，MarkWord 为："));System.out.println(ClassLayout.parseInstance(o).toPrintable());}synchronized (b) {System.out.println(("主线程再次进入同步块，并且调用hashcode方法，MarkWord 为："));b.hashCode();System.out.println(ClassLayout.parseInstance(b).toPrintable());}}

然后我们来观察下执行的结果：

/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" 并发编程03.查看内存布局信息.MarkWordDemo_4Connected 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未进入同步块，MarkWord 为：java.lang.Object object internals:OFFSZTYPE DESCRIPTIONVALUE08(object header: mark)0x0000000000000001 (non-biasable; age: 0)84(object header: class)0xf80001e5 124(object alignment gap)Instance size: 16 bytesSpace losses: 0 bytes internal + 4 bytes external = 4 bytes total未进入同步块，MarkWord 为：java.lang.Object object internals:OFFSZTYPE DESCRIPTIONVALUE08(object header: mark)0x0000000000000005 (biasable; age: 0)84(object header: class)0xf80001e5 124(object alignment gap)Instance size: 16 bytesSpace losses: 0 bytes internal + 4 bytes external = 4 bytes total进入同步块，MarkWord 为：java.lang.Object object internals:OFFSZTYPE DESCRIPTIONVALUE08(object header: mark)0x00007fe8a5009805 (biased: 0x0000001ffa294026; epoch: 0; age: 0)84(object header: class)0xf80001e5 124(object alignment gap)Instance size: 16 bytesSpace losses: 0 bytes internal + 4 bytes external = 4 bytes total新线程获取锁，MarkWord为：java.lang.Object object internals:OFFSZTYPE DESCRIPTIONVALUE08(object header: mark)0x000070000ba03908 (thin lock: 0x000070000ba03908)84(object header: class)0xf80001e5 124(object alignment gap)Instance size: 16 bytesSpace losses: 0 bytes internal + 4 bytes external = 4 bytes total主线程再次查看锁对象，MarkWord为：java.lang.Object object internals:OFFSZTYPE DESCRIPTIONVALUE08(object header: mark)0x0000000000000001 (non-biasable; age: 0)84(object header: class)0xf80001e5 124(object alignment gap)Instance size: 16 bytesSpace losses: 0 bytes internal + 4 bytes external = 4 bytes total主线程再次进入同步块，MarkWord 为：java.lang.Object object internals:OFFSZTYPE DESCRIPTIONVALUE08(object header: mark)0x0000700009f87980 (thin lock: 0x0000700009f87980)84(object header: class)0xf80001e5 124(object alignment gap)Instance size: 16 bytesSpace losses: 0 bytes internal + 4 bytes external = 4 bytes total主线程再次进入同步块，并且调用hashcode方法，MarkWord 为：java.lang.Object object internals:OFFSZTYPE DESCRIPTIONVALUE08(object header: mark)0x00007fe8a51391ea (fat lock: 0x00007fe8a51391ea)84(object header: class)0xf80001e5 124(object alignment gap)Instance size: 16 bytesSpace losses: 0 bytes internal + 4 bytes external = 4 bytes totalDisconnected from the target VM, address: '127.0.0.1:63267', transport: 'socket'Process finished with exit code 0
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