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What is the operation principle of JVM? I believe many inexperienced people are at a loss about it. Therefore, this paper summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

Brief Analysis of 1.JVM:

As a Java user, it is also necessary to master the architecture of JVM.

Speaking of Java, we first think of Java programming language, but in fact, Java is a technology, which consists of four aspects: Java programming language, Java class file format, Java virtual machine and Java application programming interface (Java API). Their relationship is shown in the following figure:

Java platform is built by Java virtual machine and Java application program interface, and Java language is the channel to enter this platform. Programs written and compiled in Java language can run on this platform. The structure of the platform is shown in the following figure:

The runtime environment represents the Java platform. Developers write Java code (.java file) and compile it into bytecode (.class file). Then the bytecode is loaded into memory, and once the bytecode enters the virtual machine, it is interpreted and executed by the interpreter or selectively converted into machine code by the just-in-time code generator.

JVM has a clear task in its life cycle, which is to run the Java program, so when the Java program starts, an instance of JVM is generated; when the program ends, the instance disappears. In the structure of the Java platform, we can see that the Java virtual machine (JVM) is in the core position, which is the key that the program has nothing to do with the underlying operating system and hardware. Below it is the port interface, which consists of two parts: the adapter and the Java operating system, in which the part that depends on the platform is called the adapter; JVM is implemented on the specific platform and operating system through the port interface At the top of JVM is the basic class library and extended class library of Java and their API. The applications (application) and Mini Program (Java applet) written by Java API can run on any Java platform without considering the underlying platform. It is because the Java virtual machine (JVM) separates the program from the operating system, thus realizing the platform independence of Java.

Below we from the basic concept of JVM and transportation process of these two aspects to conduct in-depth study of it.

Basic concepts of 2.JVM

(1) basic concepts:

JVM is a hypothetical computer that can run Java code, including a set of bytecode instructions, a set of registers, a stack, a garbage collection, a heap, and a storage method domain. JVM runs on top of the operating system and has no direct interaction with the hardware.

(2) running process:

We all know that the Java source file, through the compiler, can produce the corresponding .Class file, that is, the bytecode file, which is compiled into machine code on a specific machine through the interpreter in the Java virtual machine.

That is, as follows:

① Java source file-- > compiler-- > bytecode file

② bytecode file-- > JVM-- > machine code

The interpreter of each platform is different, but the virtual machine is the same, which is why Java can cross-platform. When a program runs from the beginning, the virtual machine begins to instantiate, and multiple virtual machine instances exist when multiple programs are started. If the program exits or closes, the virtual machine instance dies and the data cannot be shared among multiple virtual machine instances.

(3) three kinds of JVM:

HotSpot of ① Sun

JRockit of ② BEA

J9 JVM of ③ IBM

In JDK1.7 and before, we all use Sun's HotSpot, but as both Sun and BEA are acquired by oracle, jdk1.8 will use the essence of Sun's HotSpot and BEA's JRockit to form jdk1.8 's JVM.

The architecture of 3.JVM

(1) Class Loader class loader

Responsible for loading .class files, the class file has a specific file tag at the beginning of the file, and ClassLoader is responsible for loading the class file, etc., and whether it can be run or not is decided by Execution Engine.

① locates and imports binary class files

② verifies the correctness of the imported class

③ allocates initialization memory for classes

④ helps parse symbolic references.

(2) Native Interface local API:

The role of the local interface is to integrate different programming languages for Java, its original intention is to integrate the native + program, when Java was born, when it ran rampant, if you want to have a foothold, you must call the native + program, so you have to open up a special area in memory to deal with the code marked native. Its specific approach is to register the native method in Native Method Stack and load native libraies when Execution Engine is executed.

At present, this method is used less and less, unless hardware-related applications, such as driving printers through Java programs, or managing production devices in Java systems, are rare in enterprise applications.

Because now the communication between heterogeneous fields is very developed, for example, you can use Socket communication, you can also use Web Service and so on.

(3) Execution Engine execution engine: executes the instructions of the package in the method that loads the class, that is, the method.

(4) Runtime data area running data area:

Virtual machine memory or Jvm memory, open up a piece of memory in the whole computer memory to store objects, variables, etc., etc., run-time data is divided into many cells, namely: method area, virtual machine stack, local method stack, heap, program counter.

Detailed description of 4.JVM data operation area (stack tube operation, stack tube storage):

Description: JVM tuning is mainly to optimize the Heap heap and Method Area method area.

(1) Native Method Stack local method stack

It does this by registering the native method in Native Method Stack and loading native libraies when the Execution Engine is executed.

(2) PC Register program counter

Each thread has a program calculator, which is a pointer to the method bytecode in the method area (the next instruction code to be executed). The next instruction is read by the execution engine, which is a very small memory space. almost negligible.

(3) Method Area method area

The method area is shared by all threads, all fields and method bytecodes, and some special methods such as constructors, and the interface code is also defined here. To put it simply, the information about all defined methods is stored in this area, which belongs to the shared area.

Static variable + constant + class information + runtime constant pool exists in the method area, and instance variables are stored in heap memory.

(4) Stack stack

What is the ① stack?

Stack is also called stack memory, in charge of the Java program running, is created when the thread is created, its life cycle is to follow the life of the thread, the thread ends the stack memory will be released, there is no garbage collection problem for the stack, as long as the thread ends the stack on Over, the life cycle is consistent with the thread, is private to the thread.

Variables of basic types and reference variables of objects are allocated in the stack memory of the function.

What does the ② stack store?

There are three main types of data stored in the stack frame:

Local variables (Local Variables): input and output parameters and variables within a method

Stack operation (Operand Stack): record the operation of leaving and entering the stack.

Stack frame data (Frame Data): including class files, methods, etc.

Operation principle of ③ stack

The data in the stack exists in the format of Stack Frame, which is a memory block, a data set, a data set about methods and runtime data. When a method An is called, a stack frame F1 is generated and pushed into the stack, and method A calls method B, so the stack frame F2 is also pushed into the stack, and method B calls the C method, so the stack frame F3 is also pressed into the stack. After the execution is finished in turn, first eject and then enter the .F3 stack frame, then eject the F2 stack frame, and then pop the F1 stack frame.

Follow the "first in, last out" / "last in, first out" principle.

(5) Heap reactor

Heap is the largest area in JVM, and the applied objects and data all exist in this area. This area is also shared by threads, and it is also the main recycling area of gc. There is only one heap memory in a JVM instance, and the size of heap memory can be adjusted. After the class loader reads the class file, it needs to put the classes, methods, and constants in the heap memory to facilitate execution by the executor. Heap memory is divided into three parts:

The newborn area is the area where classes are born, grow and die, where a class is generated, applied, and finally collected by the garbage collector to end life. The newborn zone is divided into two parts: Eden space and Survivor pace. All the classes are new in Eden. There are two survival areas: zone 0 (Survivor 0 space) and zone 1 (Survivor 1 space). When the space in the Garden of Eden runs out, the program needs to create objects again, and JVM's garbage collector will Minor GC the Garden of Eden, moving the remaining objects in the Garden of Eden to Survivor 0. If the surviving area 0 is also full, garbage collection will be carried out in the area, and then moved to area 1. What if 1 is full? And then move to the nursing area. If the nursing area is also full, then Major GC (FullGCC) will be generated at this time to clean up the memory of the nursing area. If it is found that the object cannot be saved after performing Full GC in the nursing area, an OOM exception "OutOfMemoryError" will be generated.

If a java.lang.OutOfMemoryError: Java heap space exception occurs, the Java virtual machine does not have enough heap memory. There are two reasons:

The heap memory setting of the a.Java virtual machine is not enough and can be adjusted by the parameters-Xms and-Xmx.

b. A large number of large objects are created in the code and cannot be collected by the garbage collector for a long time (there are references).

The nursing area is used to store JAVA objects filtered from the newborn area, and pool objects are generally active in this area.

The permanent storage area is a resident memory area, which is used to store the Class,Interface metadata carried by JDK itself, that is, it stores the class information necessary for the running environment. The data loaded into this area will not be reclaimed by the garbage collector. Closing JVM will release the memory occupied by this area.

If java.lang.OutOfMemoryError: PermGen space appears, the Java virtual machine does not have enough memory settings for the permanent Perm. There are two reasons:

a. A large number of third-party jar packages need to be loaded to start the program. For example, too many applications are deployed under one Tomcat.

b. A large number of classes generated by dynamic reflection are constantly loaded, resulting in the Perm area being occupied.

Description:

Jdk1.6 and before: constant pool allocation in the permanent generation.

Jdk1.7: yes, but it has been gradually replaced permanently.

Jdk1.8 and after: none (java.lang.OutOfMemoryError: PermGen space, this error will not appear in JDK1.8).

Description: objection to method area and heap memory:

In fact, the method area, like the heap, is a shared memory area among threads, which is used to store virtual machine loaded: class information + ordinary constants + static constants + compiled code by the compiler, and so on. Although the JVM specification describes the method area as a logical part of the heap, it also has an alias called Non-Heap (non-heap), which is intended to be separated from the heap.

For HotSpot virtual machines, many developers are accustomed to calling the method area "Parmanent Gen", but they are strictly different in nature, or use permanent generation to implement the method area, which is an implementation of the method area. In the version of jdk1.7, the string constant pool that was originally placed in the permanent generation has been removed.

Constant pool (Constant Pool) is a part of the method area. In addition to the version, field, method, interface and other description information of the class, another piece of information in the Class file is the constant pool, which will be stored in the runtime pool of the method area after the class is loaded.

5. Brief introduction of heap memory tuning

Code testing:

Public static void main (String [] args) {

Long maxMemory = Runtime.getRuntime () .maxMemory ()

Long totalMemory = Runtime. GetRuntime () .totalMemory ()

System.out.println ("MAX_MEMORY =" + maxMemory + "(bytes)," + (maxMemory/ (double) 1024ax 1024) + "MB")

System.out.println ("TOTAL_ MEMORY =" + totalMemory + "(Bytes)" + (totalMemory/ (double) 1024ax 1024) + "MB")

Note: enter "- XX:+PrintGCDetails" in Run as-> Run Configurations to view the heap memory operation schematic:

(1) in jdk1.7:

(2) in jdk1.8:

6. Automatically trigger garbage collection through parameter settings:

Public class JVMTest {

Public static void main (String [] args) {

Long maxMemory = Runtime.getRuntime () .maxMemory (); / / returns the maximum amount of memory that the Java virtual machine is trying to use.

Long totalMemory = Runtime. GetRuntime () .totalMemory (); / / returns the total amount of memory in the Java virtual machine.

System.out.println ("MAX_MEMORY =" + maxMemory + "(bytes)," + (maxMemory/ (double) 1024ax 1024) + "MB")

System.out.println ("TOTAL_ MEMORY =" + totalMemory + "(Bytes)" + (totalMemory/ (double) 1024ax 1024) + "MB")

While (true) {

}

}

}

Enter the setting "- Xmx8m-Xms8m-xx:+PrintGCDetails" in Run as-> Run Configurations to see how the garbage collection mechanism works:

After reading the above, have you mastered how JVM works? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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