How much do you know about JVM memory management

JVM memory management How much do you know

1、 summary

​  “Java Virtual machine is executing Java In the process of the program, the memory it manages will be divided into several different data areas . These areas have their own uses , And when it was created and destroyed , Some regions exist with the start of virtual machine process , Some areas are created and destroyed depending on the start and end of the user thread .”

Java The data area when the virtual machine is running ：

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​   Thread isolated data area ：

• Virtual machine stack （VM Stack）
• Native Method Stack （Native Method Stack）
• Program counter （Program Counter Register）

​   Data area shared by threads ：

• Method area （Method Area）
• Pile up （Heap）

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2、Java Memory area

• Program counter （Program Counter Register）

  ​   Program counter yes For storing the next one （Java virtual machine ） Address of the unit where the bytecode instruction is located The place of , Memory of this area Belong to “ Thread private ” Memory .

  ​   If Threads What is being implemented is a Java Method , The value of this counter is executing （Java virtual machine ） The address of a bytecode instruction ; If Threads What is being executed is local （Native） Method , Then the value of this counter is null .

• Virtual machine stack （VM Stack）

  ​   Virtual machine stack yes To hold Stack frame （Stack Frame） The place of , Whenever a （Java） When the method is executed ,Java virtual machine It creates a Stack frame , Stack frame It stores Local variable table 、 The stack of operands 、 Dynamic connection 、 Method exit information , One （Java） Method from call To completion of enforcement The process of Corresponding Stack frame Push To Out of the stack The process of , Memory of this area Belong to “ Thread private ” Memory .

• Native Method Stack （Native Method Stack）

  ​   Native Method Stack With the function of Virtual machine stack be similar , The only difference is Virtual machine stack In the service of Virtual machine execution Java Method , Native Method Stack In the service of The virtual machine executes local methods , Memory of this area Belong to “ Thread private ” Memory .

• Pile up （Heap）

  ​   Pile up yes To hold （Java） Object instances The place of , Memory of this area Belong to “ Thread sharing ” Memory .

  ​   Pile up yes Garbage collector Managed memory area , So it's also called “GC Pile up ”.

  ​   The size of the heap Can pass （Java virtual machine ） Parameters -Xmx and -Xms To set .

• Method area （Method Area）

  ​   Method area It's for storing Type information loaded by the virtual machine 、 Constant 、 Static variables 、 Real time compiler compiled code cache and other data The place of , The memory of this area Belong to “ Thread sharing ” Memory .

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• Runtime constant pool （Runtime Constant Pool）

  ​   Runtime constant pool yes Method area Part of , To hold Various generated during compilation Literal And Symbol reference .

• Direct memory （Direct Memory）

  ​   Direct memory Does not belong to Java Memory area , But this part of memory is also used frequently , for example ： adopt Native The function library directly allocates Out of heap memory , Reduce data transmission loss , Improve performance .

  ​   Direct memory Only limited by local memory .

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3、Java object

3.1、 Object creation

​   stay Java In language , Creating an object requires only one keyword new , But in Java virtual machine in , To create an object, you need to go through the following steps .

​  1、 When Java virtual machine Encountered a bytecode new When the command , First of all, I'll check The parameters of this instruction Is it possible to be in Constant pool To A symbolic reference to a class , And check this The class represented by the symbolic reference whether Has been loaded 、 Parsed and initialized , If not, execute first Class loading process of this class .

​  2、 After the class load check passes , virtual machine adopt “ Pointer collision ”（ or “ Free list ”） Distribution method stay Pile up In Chinese, it means This object allocates memory space .

​  3、 After memory space allocation , virtual machine Must be Allocated memory space （ But it doesn't include the object head ） All initialized to Zero value .

​  4、 Next , virtual machine It will also help object Make the necessary Settings , Such as ： The object is an instance of which class 、 How can I find the metadata information for a class 、 Object's hash code （ Postpone to Object:hashCode() Call to And then we do the calculation ）、 Object's GC Generational age, etc , This information is stored in In the object header of the object .

​  5、 thus , From a virtual machine perspective , A new object has been born , But from Java From a procedural point of view , Object creation just started （ The constructor has not been executed yet ）, Next is Execute constructor （ In the virtual machine, it is <init>() Method ） Finish right Object initialization , To complete formally Object construction .

3.2、 Memory layout of objects

​   object stay Pile up The memory layout in can be divided into three parts ：

• Object head （Header）
  • The running data of the object itself ： Hash code 、GC Generational age 、 Lock status flag, etc
  • Type a pointer ： Point to Object type metadata （ namely Is an instance of which class ）
• The instance data （Instance Data）
  • Field contents
• Add to it （Padding）
  • Place holder

3.3、 Object access location

​  Java When programs use objects , Will pass Stack Upper reference Data to operate Pile up Specific objects in , and reference（ quote ） Access location Pile up Objects in can be divided into two ways ：

• Handle access

  ​   Pile up Partition a piece of memory as a handle pool （ Handle contains Object instance data address And Type data address ）,reference What's stored in is Object's handle address .

  ​   advantage ：reference A stable handle address is stored in , Only the instance data pointer in the handle will be changed when the object is moved .

• Direct pointer access

  ​  reference Store directly in Object address .
  

    advantage ： Compared with the handle mode Save the time cost of a pointer location , Improved access speed .

3.4、 References to objects

• Strong citation （Strongly Reference）

  ​   finger Reference assignment in program code （ Object obj = new Object() ）, as long as Strong reference relationship There is still , Garbage collector Will never be recycled The object of reference .

• Soft citation （Soft Reference）

  ​   finger Some of them are useful 、 But not necessarily , Will happen in the system Memory overflow exception front , Garbage collector These objects will be recycled .

  ​  JDK1.2 after , Provide SoftReference class Realization Soft citation .

• Weak reference （Weak Reference）

  ​   finger Some non essential objects , When Garbage collector When starting work , Whether the current memory is sufficient or not , These objects will be recycled .

  ​  JDK1.2 after , Provide WeakReference class Realization Weak reference .

• Virtual reference （Phantom Reference, also called “ Ghost quotes ” or “ Phantom reference ”）

  ​   Whether an object has a virtual reference , It doesn't affect their survival time at all , Nor can virtual references be used to get an object instance , The only purpose of its existence is ： In order to be able to be Garbage collector Receive a system notification when recycling .

  ​  JDK1.2 after , Provide PhantomReference class Realization Virtual reference .

3.5、 The survival of objects

​   Be an object “ Death ” when ,GC（Garbage Collection, garbage collection ） The object will be cleared and Reclaim the memory space allocated to the object . that ,GC How to judge whether an object is “ Death ” What about it ？

​  GC Determine whether an object is “ Death ” There are usually two algorithms ：

• Reference counting algorithm

  ​   stay object Add a Reference counter , Whenever there's a place to quote it , Counter value plus one ; Whenever a reference to a place fails , The counter value minus one ; The counter value at any time is zero The object of is “ Dead ” The object of （ namely This object can no longer be used ）.

  • advantage ： The principle of simple , High judgment efficiency
  • shortcoming ： Take up extra memory space 、 Cannot solve the problem of circular reference between objects

• Reachability analysis algorithm

  ​   With “GC Roots”（ Root object ） As the starting node set , Start with these nodes , Search down... Based on reference relationships , The path taken by the search process is called “ References to the chain （Reference Chain）”, If An object To “GC Roots” There is no References to the chain Connected to a （ perhaps In the words of graph theory, it is from “GC Roots” To This object is unreachable ） when , This object “ Dead ”（ namely This object can no longer be used ）.

  • advantage ： It solves the problem of circular reference between objects
  • shortcoming ： Compare with Reference counting algorithm Judge efficiency It's not that high

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  Expand ：Java Medium “GC Roots”

  • Virtual machine stack （ Local variables in stack frames ） in Referenced object , Such as ： Currently running method Used in Parameters 、 local variable 、 Temporary variables, etc
  • Method area Class static properties Referenced object , Such as ：Java class Reference type static variable of
  • Method area Constant Referenced object , Such as ： The string reference in the pool
  • Local method stack JNI（Native Method ） Referenced object
  • Java virtual machine Internal references , Such as ： Basic data type object Class object 、 Some resident exception objects （NullPointException、OutOfMemoryError）、 system class loader
  • All are locked in sync （synchronized keyword ） The object of holding
  • reflect Java virtual machine The internal situation JMXBean、JVMTI Callback registered in 、 Local code cache, etc
  • Different garbage collectors and Different reclaimed memory areas Medium “ Temporary ” object

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• Object's “ Probation ”：

​   When an object is in Reachability analysis algorithm It is determined as When an object is unreachable , The object will enter “ Probation ” Stage ： Detect whether it is necessary for the object to execute finalize() Method .

​   If the object is not overridden finalize() Method （ or finalize() Method has been called by the virtual machine ）, be virtual machine Think that the object It's not necessary finalize() Method , Really announce its “ Death ”;

​   otherwise , virtual machine Think that the object It is necessary to carry out finalize() Method , And put the object into F-Queue In line , Then the virtual machine Automatically created 、 Low scheduling priority Of Finalizer Threads To carry out its finalize() Method （ there “ perform ” The virtual opportunity triggers the method to run , But it doesn't promise to wait for it to run out ）.

• Object's “ be reborn ”：

​   When the object is finalize() In the method Re associate with any object on the reference chain , Such as ： Put yourself （this keyword ） Assign to a class variable or a member variable of an object , namely This object gets “ be reborn ”（ Removed from the “ About to be recycled ” Set ）.

​   contrary , If the object is in finalize() In the method Failed to re associate with any object on the reference chain , The object will also face “ Death ”.

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4、 Garbage collection algorithm

4.1、 Generational collection theory

​   Generational collection is called theory , In essence, it's a set of rules of thumb that fit the actual situation of most programs , It is based on two generational hypotheses and a rule of thumb .

• Weak generational Hypothesis （Weak Generational Hypothesis）： The vast majority of objects are born and perished

• Strong generational Hypothesis （Strong Generational Hypothesis）： The more times you go through the garbage collection process, the more difficult it is for an object to die

• Cross generational citation Hypothesis （Intergenerational Reference Hypothesis）： Cross generation references account for only a very small number of references compared to the same generation references

​   be based on Generational collection theory , Garbage collector The consistent design principle is ： The collector should put Java Pile up different areas , Then the objects will be recycled according to their age （ The number of times an object has survived the garbage collection process ） Allocated to different areas to store .

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Java Generation of the heap

• The new generation （Young Generation）
• Old age （Old Generation）

Generational collection

• Part of the collection （Partial GC）：
  • Cenozoic collection （Minor GC / Young GC）： Only garbage collection for the new generation
  • Old age collection （Major GC / Old GC）： Garbage collection for the elderly only
  • Mixed collection （Mixed GC）： Yes Garbage collection is carried out throughout the Cenozoic , Also on the Garbage collection is carried out in some old times
• Collect the whole pile （Full GC）： To the whole Java Pile up and Method area Garbage collection

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4.2、 Mark - Clear algorithm

​   The algorithm is divided into “ Mark ” and “ eliminate ” Two phases ： First mark all objects to be recycled , After the tag is done , Recycle all marked objects in a unified way （ Or uniformly recycle all unmarked objects ）.

​   shortcoming ： Creating debris space .

4.3、 Mark - Copy algorithm

​   Divide memory into two equal sized blocks according to capacity , Use only one piece at a time , When this block of memory runs out , Copy the existing object to another piece , Then clean up the used memory space at one time .

​   shortcoming ： Halve available memory .

4.4、 Mark - Sorting algorithm

​   Marking process of Algorithm And “ Mark - Clear algorithm ” Agreement , The sorting process is ： Move all living objects to one end of memory space , Then clean up the memory outside the boundary .

​   shortcoming ： Move Big object It may cause a long pause .

All the above contents are from the right Teacher zhou zhiming's 《 In depth understanding of Java virtual machine 》 Summary of some contents of .
My summary is compared with the content of this book , Nothing is equal to the water drop and the river , I sincerely recommend you to read this book carefully .
The above content is only for personal learning .