JVM(essential)

• JVM JRE JDK

1. JVMabstract machine:platform dependent 
2. JVMfunction:1 load codes;2verify codes(strictly typed)--compile error; 3excute the codes;4provide runtime environment
3. runtime environment:actual implementation of JVM, JVM is in jre(physically present), jre contains libraries and other files JVM uses.
4. JDK contains JRE+development tools(like javac),also physically present

class loader + runtime data areas + excution engine 

• class loader{load + link + initialize}

• load{ bootstrap class loader<- extension class loader<-application class loader，查找.class文件顺序 }

1. 通常class file not found
2. load from .class or .jar file which contains .class file
3. loader  implement不同则接收不同data source
4. bootstrap class loader: load rt.jar
5. extension class loader:load form jre/lib/ext folder
6. application class loader:load from CLASSPATH -cp
7. CLASSPATH and settings

1. 1. .class重名 ，-cp 选择在CLASSPATH中第一个出现的.class文件
   2. 而-Xmx设置则选择后者 java -Xmx128m -Xmx256m Hello.class 
   3. jinfo -flag MaxHeapSize pid 查看

• link{verify + prepare +resolve}

1. verify byte codes
2. memory allocation for class vars to default value 
3. resolve： all symbolic references inside the current class(references to其他classes 或 to values constants pool)
4. resolve通常产生class def not found，即class文件中所有对其他class 或methods的引用not found

• initialize

1. static block
2. set values for static vars(vars to initalized value)

• runtime data areas

Method + Heap + Java Stacks + PC registers +Native method Stacks

• method area

1. stored metadata corresponding to class（reflectionAPI所为即访问这里的metadata数据）
2. 比如static var ，constants pool等class level的data  分配给JVM的内存，-XX:MaxPermSize 默认值64MB
3. 内存不够 java.lang.OutOfMemoryError:PermGen space...
4. !!java8 中为meta space并且没有限制 将PermGen space移到了native os的一个分开的内存

• heap

1. object data, 每一次new 或arrys等
2. -Xms -Xmx 最小最大的内存 分配

• programcounter register

1. 对每个thread有一个program counter
2. counter则是指向下一instruction的指针

• stack
1. 每个thread 有一个stack
2. 将thread的不同methods push到stack中
3. 如m1 call m2那么将在m1基础上push m2到stack中，return则将m2 pop out
4. 数据如 parameters ， parameter array，local vars， return values， operant stack(暂存区进行操作)
5. java.lang.StackOverflowError    -Xss

• native method stack

1. 如 call .dll /.so文件在本地 lib

每个JVM只有一个method area和heap，但thread级是stack，如果彼此不交互则threadsafe

• excution engine

{interpreter + JIT compiler+hostspot profiler+garbage collector}

•  interpreter

1. interpret the current bytecode instruction

• Just-In-Time compiler &hotsot profiler

1. 对于重复的指令则不再重复用interpreter而是保存起来
2. hotsot 根据interpreter行为特点的观察，发现不断地重复执行相同指令，让JIT compiler保存

• GC

1. clean up unused calsses objects and memory areas

• JNI java native interface  (medium for java method calls for native methods . provide info about native library to JVM

JVM(essential)

原文：http://www.cnblogs.com/yumanman/p/7612454.html