java虚拟机jvm启动后java代码层面发生了什么?
java虚拟机jvm启动后java代码层面发生了什么?
java代码在被编译后可以被jdk提供的java命令进行加载和运行, 在我们的程序被运行起来的时候,都发生了什么事情, 下面就来探究下这个问题, 这个问题被拆成了两个问题, 第一个问题用来确定发生了哪些事情, 第二个问题用来确定这些事情是如何进行的.
- java进程里面都发生了哪些活动?
- 这些活动在java代码(反编译或者是源码)级别有所体现吗?
0001 寻找验证的方式
当我在探究上面两个问题时, 我想了很多方式去探究这两个问题,下面是我想到的方法
- java官方文档? 没有找到
- 搜索引擎? 没有找到
- idea中debug?
有点线索
在idea中打上断点然后debug时, 我发现了一个东西,以下是idea中的debug截图
idea调试面板
看到了这个关于线程和调用堆栈的线索, 那么java代码层面中发生了哪些事情就好确定了
通过线程名字可以看出这个线程主要负责的事情.
0002 确认验证的入口
idea中的debug窗口的线程选择下拉框显示了一些可以进行debug操作的线程, 那么可以断定这几个线程都有对应java代码入口, 我们可以根据这些堆栈的入口来进行代码阅读, 感兴趣的可以看看里面究竟发生了哪些事情
idea中的可debug的线程没有通过jstack命令显示的线程多, 下面是通过jstack命令dump出来的数据:
"RMI TCP Connection(2)-192.168.191.1" #18 daemon prio=5 os_prio=0 tid=0x000000001b409800 nid=0x80a4 runnable [0x000000001c93d000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
at java.net.SocketInputStream.read(SocketInputStream.java:171)
at java.net.SocketInputStream.read(SocketInputStream.java:141)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)
at java.io.BufferedInputStream.read(BufferedInputStream.java:265)
- locked <0x00000000d7520cb0> (a java.io.BufferedInputStream)
at java.io.FilterInputStream.read(FilterInputStream.java:83)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:555)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:834)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:688)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$Lambda$3/1223273742.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:687)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:
- <0x00000000d6c99530> (a java.util.concurrent.ThreadPoolExecutor$Worker)
"JMX server connection timeout 17" #17 daemon prio=5 os_prio=0 tid=0x000000001ae9e000 nid=0x8088 in Object.wait() [0x000000001c63f000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d715f378> (a [I)
at com.sun.jmx.remote.internal.ServerCommunicatorAdmin$Timeout.run(ServerCommunicatorAdmin.java:168)
- locked <0x00000000d715f378> (a [I)
at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:
- None
"RMI Scheduler(0)" #16 daemon prio=5 os_prio=0 tid=0x000000001b1c6800 nid=0x8084 waiting on condition [0x000000001c53e000]
java.lang.Thread.State: TIMED_WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000000d69fbee0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078)
at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1093)
at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:
- None
"RMI TCP Connection(1)-192.168.191.1" #15 daemon prio=5 os_prio=0 tid=0x000000001ae1a800 nid=0x8080 runnable [0x000000001c43d000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
at java.net.SocketInputStream.read(SocketInputStream.java:171)
at java.net.SocketInputStream.read(SocketInputStream.java:141)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)
at java.io.BufferedInputStream.read(BufferedInputStream.java:265)
- locked <0x00000000d6ddc2e0> (a java.io.BufferedInputStream)
at java.io.FilterInputStream.read(FilterInputStream.java:83)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:555)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:834)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:688)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$3/1223273742.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:687)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:
- <0x00000000d6c93ac0> (a java.util.concurrent.ThreadPoolExecutor$Worker)
"RMI TCP Accept-0" #14 daemon prio=5 os_prio=0 tid=0x000000001a833800 nid=0x8074 runnable [0x000000001c23e000]
java.lang.Thread.State: RUNNABLE
at java.net.DualStackPlainSocketImpl.accept0(Native Method)
at java.net.DualStackPlainSocketImpl.socketAccept(DualStackPlainSocketImpl.java:131)
at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)
at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:199)
- locked <0x00000000d6be9bb0> (a java.net.SocksSocketImpl)
at java.net.ServerSocket.implAccept(ServerSocket.java:545)
at java.net.ServerSocket.accept(ServerSocket.java:513)
at sun.management.jmxremote.LocalRMIServerSocketFactory$1.accept(LocalRMIServerSocketFactory.java:52)
at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:405)
at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:377)
at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:
- None
"Service Thread" #12 daemon prio=9 os_prio=0 tid=0x000000001a653800 nid=0x3be8 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"C1 CompilerThread2" #11 daemon prio=9 os_prio=2 tid=0x000000001a63f000 nid=0x70b8 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"C2 CompilerThread1" #10 daemon prio=9 os_prio=2 tid=0x000000001a5ed800 nid=0x7898 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"C2 CompilerThread0" #9 daemon prio=9 os_prio=2 tid=0x000000001a5ec800 nid=0x79a4 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"JDWP Command Reader" #8 daemon prio=10 os_prio=0 tid=0x0000000018458800 nid=0x7520 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"JDWP Event Helper Thread" #7 daemon prio=10 os_prio=0 tid=0x0000000018455000 nid=0x7c44 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"JDWP Transport Listener: dt_socket" #6 daemon prio=10 os_prio=0 tid=0x0000000018449000 nid=0x7a50 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x00000000197b3000 nid=0x3250 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001842a800 nid=0x4c8c runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLELocked ownable synchronizers:
- None
"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x00000000037bd800 nid=0x764c in Object.wait() [0x000000001976f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d5d88ed0> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
- locked <0x00000000d5d88ed0> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)Locked ownable synchronizers:
- None
"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x00000000037bc000 nid=0x3c8c in Object.wait() [0x000000001966e000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d5d86bf8> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
- locked <0x00000000d5d86bf8> (a java.lang.ref.Reference$Lock)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)Locked ownable synchronizers:
- None
"main" #1 prio=5 os_prio=0 tid=0x00000000036c4000 nid=0x1cb0 runnable [0x00000000035bf000]
java.lang.Thread.State: RUNNABLE
at java.io.FileInputStream.readBytes(Native Method)
at java.io.FileInputStream.read(FileInputStream.java:255)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)
at java.io.BufferedInputStream.read(BufferedInputStream.java:265)
- locked <0x00000000d5de89a8> (a java.io.BufferedInputStream)
at com.TaskTest.main(Test.java:179)Locked ownable synchronizers:
- None
"VM Thread" os_prio=2 tid=0x00000000183d8000 nid=0x6d38 runnable
"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00000000036e2800 nid=0x7e88 runnable
"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00000000036e4000 nid=0x35e4 runnable
"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00000000036e5800 nid=0x4ed4 runnable
"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00000000036e7000 nid=0x702c runnable
"VM Periodic Task Thread" os_prio=2 tid=0x000000001a69f000 nid=0x2af8 waiting on condition
JNI global references: 5253
可以看出来jstack导出的线程有些有调用堆栈, 有些没有, 是这样的, 没有堆栈的线程应该是jvm的一些核心线程, 这些核心线程提供了jvm的核心功能, 比如垃圾回收, 即时编译等. 带有堆栈的这些线程都是由java代码作为入口构建出来的, 所以我们能根据这些入口去查看里面的具体逻辑, 这些线程中有一个线程main, 这个线程就是为带有main方法入口的java应用构建出来的, 这个线程主要执行动作就是java应用由入口方法所触发的程序逻辑.
另外带有堆栈信息的这些线程, 在执行其自身代码的逻辑之外, 还会被jvm用于执行类加载这个动作. 如何验证这个类加载的动作呢, 如果你用的idea, 在设置中开启debug时进入类加载器断点, idea默认不开启这个动作, 然后把断点打点到类加载器的loadClass这个方法中, 给出以下具体位置:sun.misc.Launcher.AppClassLoader#loadClass, 这只是咱们项目代码中的定义类型的类加载器.
那如何确定是某个类的类加载器呢, 大家可以在自己的先获取类的类对象, 根据类对象的方法getClassLoader就可以确认类加载器了, 对于AppClassLoader这个类加载器大家肯定不会陌生, 比较有名双亲委派模型就是由这个类作为入口实现的, 大家有兴趣的可以去扒下代码, 验证下自己读过的文章是不是和代码一致. 最后以debug模式启动你的main代码, 你就可以命中类加载器中的代码了. 好了, 好好琢磨琢磨这些代码逻辑吧.
下次再见
0003 网络资源
下面是一些本文的相关资源
JVM 内部运行线程介绍
https://www.cnblogs.com/williamjie/p/9389349.html
各个工具如何查找加载类
https://docs.oracle.com/javase/8/docs/technotes/tools/findingclasses.html
java虚拟机如何查找加载类
https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html
