Detailed explanation of Java runtime data area and class structure

Time:2021-9-23

Java runtime data area

Java runtime data area can be divided into: method area, virtual machine stack, local method stack, heap and program counter

Thread private: virtual machine stack, local method stack, program counter

Thread sharingMethod area, heap:

Program counter

A small memory space. It is the line number indicator of bytecode executed by the current thread. It is the indicator of program control flow. Basic functions such as branch, loop, jump, exception handling and thread recovery need to rely on this counter.

Each thread has a separate program counter.

Java virtual machine stack

Threads are private and have the same life cycle as threads.

When each method is executed, the Java virtual machine creates a stack frame for storageLocal variable table, operand stack, dynamic link, method exitOther information

dynamic link: the process of converting a symbol into a direct reference during operation is dynamic connection (the corresponding static connection in advance refers to the conversion of a static symbol reference into a direct reference during class loading).

Native Method Stack

It is similar to the Java virtual machine stack, but it works on the local native method.

Java heap

A memory area shared by threads to store object instances. “Almost” all objects are allocated in the heap.

Due to timely compilation, especially the increasing demand for escape analysis technology, objects are not necessarily allocated in the heap (possible)On stack allocationandscalar replacement )。

The Java heap can be divided into multiple thread private allocation buffers (tlabs) to improve the efficiency of object allocation. This TLAB only ensures that the thread can allocate here, but all threads can access it.

Method area

Thread sharing, storing the data loaded by the virtual machineType informationconstantStatic variableCode cache compiled by instant editorAnd other data.

Runtime constant pool for method area: store the constant pool table in the class file loaded in the class loader.

Java object memory allocation

Bytecode new instruction – > check constant pool – > classloader (load, connect (check, prepare, parse), initialization)

After checking, it is time to allocate memory for the new object. Allocation policy:

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escape analysis

Whether the scope of the analysis object is in this method. If it is only in this method, it can be allocated on the stack. After escape analysis JDK7, it is enabled by default.

The object of new is not necessarily in the heap. It may be allocated and replaced by scalar on the stack

On stack allocation: one of the JVM tuning methods. If the method object does not escape, it can be allocated on the stack. Its life cycle is consistent with the method call to reduce the pressure of GC.

scalar replacement : if there is no escape from the object, the JVM may not create the object, but decompose the object variable into several member variables for replacement, so that it can be allocated on the stack frame or register (without continuous space), JDK7 which is enabled by default. Scalar substitution takes precedence over allocation on the stack.

TLAB: thread allocates cache locally (also in heap)

When allocating memory in Eden, if multiple threads allocate memory at the same time, it will cause pointer collision. In order to improve the efficiency of object allocation, TLAB is used.

When a thread is initialized, it will apply for a little memory of the specified size, and only the current thread is provided for memory allocation, so that each thread has a separate space.

TLAB is a special space divided by virtual machine in Eden of heap memory.

When the TLAB does not have enough space to meet the operation, you need to re apply for a new TLAB from the current thread

Java class file structure

The file structure of class bytecode is analyzed in strict order

type name remarks
u4 magic Magic number, identifying class file format, value: 0xcafebabe
u2 minor_version Sub version number
u2 major_version Major version number, 45 -?, 57 for jdk13 and 52 for jdk8
u2 constant_pool_count Constant pool calculator
cp_info constant_pool Constant pool
u2 access_flags There are 9 access flags, including public and final. There are 16 flag bits, and each bit identifies an access flag.
u2 this_flags Class index, index value in constant pool
u2 super_class Parent index, index value in constant pool
u2 interfaces_count Interface counter
u2 interfaces Interface index collection, index value in constant pool
u2 fields_count Number of fields
field_info fields Field set, field flag (public, static, etc.), field name constant index, description constant index (type)
u2 methods_count Method counter
method_info methods Method set is similar to field set, including method flag, method name index and method description index (return type and method parameter list)
u2 attributes_count Additional attribute counter
attribute_info attributes Additional attribute collection

Constant pool

Constant pools are divided into:Literal andSymbol reference

Literal: text string, final constant value, etc

Symbol reference:

  • Fully qualified names of classes and interfaces
  • Names and descriptors of fields and methods
  • Method handle and type
  • Dynamic call points and dynamic constants

Constant pool item type:

Attribute table

Class files, field tables, and method tables can all carry their own set of attribute tables to describe information specific to certain scenarios

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The attributes (parts) are:

For example, the code attribute,

Class loading mechanism

Class loading process:

Load – > link (verify, prepare, parse) – > initialize

Load: load bytecode with classloader

Verification: verify the legitimacy of bytecode (meet the constraints)

Preparation: allocate memory for static fields of the loaded class

Resolve: resolve symbolic references to actual references.

Initialization: initializes constants and static classes

Class loader:

Start the class loader: load the most basic and important classes, such as the classes in the jar package under JRE’s lib

Extension class loader: his Frey is the startup class loader, which mainly loads relatively minor but common classes, such as jar classes under lib / ext of JRE

Application class loader: its parent class is the extension class loader, which is responsible for loading classes under the application path. (refers to the path specified by the virtual machine parameter – CP / – classpath, system variable java.class.path or environment variable classpath)

If the same byte stream is loaded by different class loaders, two different classes will be obtained.

Parental delegation mode: let the parent loader load as much as possible

Destruction of parental delegation mode:

1) If the class loaded by the upper class loader loads the class loaded by the lower class loader

Java introduces the context class loader, which can get through Frey loader to request the loading behavior of subclass loader. For example, when JNDI calls the service code.

2) OSGi hot deployment uses mesh class loading mode.

The above is my personal experience. I hope I can give you a reference, and I hope you can support developpaer.

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