Introduction to memory model of JVM

1. Pile up （Heap）

 

Heap memory is common to all threads , It can be divided into two parts ： The young and the old . Image below Perm It stands for permanent generation , But note that permanent generations are not part of heap memory , meanwhile jdk1.8 Then the permanent generation will be removed .

 

The pile is java The largest area of memory managed by a virtual machine , It is also the memory area shared by various threads , This memory area stores object instances and arrays （ But not all object instances are in the heap ）. Its size passes through -Xms( minimum value ) and -Xmx( Maximum ) Parameter setting （ The maximum and minimum values should be less than 1G）, The former is the minimum memory requested at startup , The default is the physical memory of the operating system 1/64, The latter is JVM Maximum memory that can be requested , Default to physical memory 1/4, By default, when the empty heap memory is less than 40% when ,JVM Will increase heap memory to -Xmx Specified size , It can be done by -XX:MinHeapFreeRation= To specify the ratio column ; When the free heap memory is greater than 70% when ,JVM Will reduce the size of heap memory to -Xms Specified size , It can be done by XX:MaxHeapFreeRation= To specify the ratio column , Of course, to avoid frequent adjustments at runtime Heap Size , Usually -Xms And -Xmx Set the value of to the same . Heap memory = The new generation + Old generation + Lasting generation . When we recycle , We tend to divide heap memory into new and old generations （ Size ratio 1：2）, In the Cenozoic, there are Eden and Survivor0,Survivor1 form , The ratio of the three is 8：1：1, The recycling mechanism of the new generation adopts replication algorithm , stay Minor GC When , We all leave a living area for living objects , The real area is Eden+ One of the survival areas , When our object is older than a certain age （ Default 15, It can be set by parameters ）, Will put the object into the old generation , Of course, large objects will directly enter the older generation . The recycling algorithm adopted by the older generation is the tag sorting algorithm .

2. Method area （Method Area）

Method area also known as ” Forever “, It is used to store the class information loaded by the virtual machine 、 Constant 、 Static variables 、 Is an area of memory Shared by each thread . The default minimum value is 16MB, The maximum value is 64MB（64 position JVM Because the pointer expands , The default is 85M）, Can pass -XX:PermSize and -XX:MaxPermSize Parameters limit the size of the method area . It's a continuous heap space , The permanent generation of garbage collection is with the elderly generation (old generation) Tied together , So no matter who is full , Will trigger permanent generation and old generation garbage collection . however , An obvious problem is , When JVM The loaded class information capacity exceeds the parameter -XX：MaxPermSize When the set value , Application will report OOM Error of . The parameter is passed through -XX:PermSize and -XX：MaxPermSize To set .

3. Virtual machine stack (JVM Stack)

Describe the java Memory model for method execution ： When each method is executed, a ” Stack frame ”, Used to store local variables ( Including the parameters )、 Stack operation 、 Method exit information . Each method is called to the finished procedure , This corresponds to the process of a stack frame in the virtual machine stack from push to push . The declaration cycle is the same as the thread , Threads are private . Stack frame consists of three parts ： Local variable region 、 The stack of operands 、 Frame data area . Local variable areas are organized in units of one word length 、 from 0 Array to start counting , Same as the local variable area , The stack of operands is also organized into an array of word lengths . But unlike the former , It's not accessed by index , It's accessed by pushing and pushing , It can be seen as a storage area for temporary data . In addition to the local variable area and operand stack ,java Stack frames also need some data to support constant pool parsing 、 Normal method return and exception dispatch mechanism . These data are stored in java In the frame data area of the stack frame .

Local variable table : It stores all kinds of basic data types known by the compiler 、 Object reference ( Reference pointer , Not the object itself ), among 64 A length of long and double Type of data is occupied 2 Space of local variables , The rest of the data types only account for 1 individual . The memory space required for local variables is allocated during compilation , When entering a method , How many local variables should be allocated in the stack frame is completely determined , The stack frame does not change the size space of the local variable table during runtime .

4. Native Method Stack (Native Stack)

Similar to virtual machine stack , The difference is that the virtual machine stack performs java Method service , The local method stack is for Native Method service .( Stack space is much smaller than heap space )

5. Program counter （PC Register）

It's the smallest memory area , Its function is the line number indicator of bytecode executed by the current thread , In the virtual machine model , The bytecode interpreter works by changing the value of this counter to select the next bytecode instruction to execute , Branch 、 loop 、 exception handling 、 Thread recovery and other basic functions need to rely on counters to complete .

6. Direct memory

Direct memory is not part of virtual machine memory , Neither Java Memory area defined in virtual machine specification .jdk1.4 New to China NIO, Introduced channel and buffer IO The way , It can call Native Method to allocate out of heap memory directly , This out of heap memory is native memory , Does not affect the size of heap memory .