JVM 基础知识
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什么 JVM ?
JVM 是可运行 Java 中假想的计算机,就是在真实的计算机上模拟计算机功能,包括一套字节指令,一套寄存器,一个栈,一个垃圾回收,堆,一个存储区域,JVM 是运行在操作系统之上,它与硬件没有直接的交互。
Java 源文件通过编译器,能够生产相应的 .class 文件,也就是字节码文件。而字节码又通过 Java 虚拟机中解释器,编译成特定机器上的机器码。
1. Java 源码文件 -----》 编译器 ------》字节码文件
2. 字节码文件 -----》JVM ----》 机器码
每种平台的解释器是不同的,但是实现的虚拟机是相同的,这也就是 Java 为什么能够跨平台的原因。当一个程序从开始运行,这是虚拟机开始实例化,多个程序启动就会存在多个虚拟机程序。程序退出或者关闭,虚拟机消亡,多个虚拟机实例之间数据不能共享。
Hotspot JVM 中的 Java 线程与 原生操作系统线程有直接的映射关系。当线程本地存储,缓冲区分配,同步对象,栈,程序计数器准备好之后,创建一个操作系统原生线程,Java 线程结束,原生线程将被回收。
Hotspot JVM 后台运行的系统线程主要有下面几个
列表参考
2020-01-27 18:04:11
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.40-b25 mixed mode):
"Service Thread" #10 daemon prio=9 os_prio=0 tid=0x00000000195ee800 nid=0x1bc runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C1 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x00000000195ec000 nid=0x4750 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x0000000019593000 nid=0x51e0 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x0000000019590800 nid=0x4704 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x000000001956f000 nid=0x2f7c runnable [0x0000000019e2e000]
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:170)
at java.net.SocketInputStream.read(SocketInputStream.java:141)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
- locked <0x00000000d5fadb60> (a java.io.InputStreamReader)
at java.io.InputStreamReader.read(InputStreamReader.java:184)
at java.io.BufferedReader.fill(BufferedReader.java:161)
at java.io.BufferedReader.readLine(BufferedReader.java:324)
- locked <0x00000000d5fadb60> (a java.io.InputStreamReader)
at java.io.BufferedReader.readLine(BufferedReader.java:389)
at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:64)
"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000017c2b000 nid=0x5050 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000000017be0000 nid=0x13b0 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x0000000003043000 nid=0x2d9c in Object.wait() [0x0000000018f2f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d5b86f58> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
- locked <0x00000000d5b86f58> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)
"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x0000000003040000 nid=0x539c in Object.wait() [0x0000000018e2e000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d5b86998> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:157)
- locked <0x00000000d5b86998> (a java.lang.ref.Reference$Lock)
"main" #1 prio=5 os_prio=0 tid=0x0000000002f53800 nid=0x1ea0 runnable [0x0000000002d5e000]
java.lang.Thread.State: RUNNABLE
at java.io.FileInputStream.readBytes(Native Method)
at java.io.FileInputStream.read(FileInputStream.java:255)
at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
- locked <0x00000000d5bdc1c8> (a java.io.BufferedInputStream)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
- locked <0x00000000d5efa2b8> (a java.io.InputStreamReader)
at java.io.InputStreamReader.read(InputStreamReader.java:184)
at java.io.Reader.read(Reader.java:100)
at java.util.Scanner.readInput(Scanner.java:804)
at java.util.Scanner.hasNext(Scanner.java:1339)
at algorithm.Main.main(Main.java:29)
"VM Thread" os_prio=2 tid=0x0000000017b97800 nid=0x3c78 runnable
"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002f69000 nid=0x3860 runnable
"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000002f6b000 nid=0x2ab0 runnable
"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000002f6c800 nid=0x507c runnable
"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000002f6e000 nid=0x3458 runnable
"VM Periodic Task Thread" os_prio=2 tid=0x00000000195f0000 nid=0x3814 waiting on condition
JNI global references: 33
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