Catalog

 

jvm Memory model

1. Pile up （Heap）

2. Method area

3. Virtual machine stack

4. Native Method Stack

5. Program counter

Direct memory

Frequently asked questions

1. To illustrate an object from new Start interaction with memory

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jvm Memory model

jvm The memory model can be divided into two parts , As shown in the figure below , The heap and method area are common to all thread pools , And virtual machines z Virtual machine stack , Local method stacks and program counters are thread private

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 On behalf of the permanent representative , But note that permanent generations are not part of heap memory , meanwhile jdk1.8 Later permanent generations will also 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 （ When a GC In case of occurrence Eden And a survivor The objects that survive in are placed in another piece survivor in ）, 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 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

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

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

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 .

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 .

Reproduced in

https://www.cnblogs.com/itsharehome/p/11290907.html

Frequently asked questions

1. To illustrate an object from new Start interaction with memory