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This article introduces the relevant knowledge of "how to understand JVM memory structure program counters and stacks". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Program counter

In the program count register (Program Counter Register) in JVM, the name of Register is derived from the register of CPU, which stores the field information related to instructions. CPU can only run if the data is loaded into a register.

For example:

Public int test () {

Int x = 0

Int y = 1

Return x + y

}

Virtual machine stack

Characteristics of stack

1. Basic operation

two。 Is there garbage collection in the stack?

3. What exception may be thrown on the stack? StackOverflowError? OutOfMemoryError?

4. How do I set the size of stack memory? -Xss size (i.e.-XX:ThreadStackSize)

The default is generally 512k-1024k, depending on the operating system.

Corresponding bytecode:

Public int test ()

Descriptor: () I

Flags: ACC_PUBLIC

Code:

Stack=2, locals=3, args_size=1

0: iconst_0

1: istore_1

2: iconst_1

3: istore_2

4: iload_1

5: iload_2

6: iadd

7: ireturn

LineNumberTable:

Line 7: 0

Line 8: 2

Line 9: 4

LocalVariableTable:

Start Length Slot Name Signature

0 8 0 this Lcom/alibaba/uc/TestClass

2 6 1 x I

4 4 2 y I

Why do you need to record the execution address of the current thread?

Why are PC registers thread private?

Stack frame (Stack Frame)

1. What is the relationship between the method and the stack frame?

The stack frame is a memory block and a data set that maintains all kinds of data information during the execution of the method.

two。 The internal structure of the stack frame

Local variable scale:

1. Defined as an array of numbers, mainly used to store method parameters and local variables defined in the body of the method

two。 These data types include various basic data types, object references (reference), and returnAddress types

3. Is there a thread safety problem?

4. Give an example

Public class SlotTest {

Public void localVarl () {

Int a = 0

System.out.println (a)

Int b = 0

}

Public void localVar2 () {

{

Int a = 0

System.out.println (a)

}

/ / at this time, b will reuse the slot of a

Int b = 0

}

}

5. The variables in the local variable table are also important garbage collection root nodes, as long as the objects referenced directly or indirectly in the local variable table will not be recycled.

Operand stack:

1. Operand stack, also known as expression stack, last-in, first-out.

two。 In the process of method execution, data is written to or extracted from the stack according to bytecode instructions, that is, push / pop. Index access is not used.

3. For example:

Public void testAddOperation () {

Byte I = 15

Int j = 8

Int k = I + j

}

4. Bytecode analysis:

Dynamic links:

Dynamic linking (or a method reference to a running constant pool)

1. Each stack frame contains a reference to the method to which the stack frame belongs in the runtime constant pool. The purpose of this reference is to enable code that supports the current method to implement dynamic linking.

two。 When the java source file is compiled into a bytecode file, all variable and method references are saved as symbolic references (Symbolic Reference) in the constant pool of the class file. For example, when a method calls another method, it is represented by symbolic references to the method in the constant pool, then the purpose of dynamic linking is to convert these symbolic references into direct references to the calling method.

3. Why do I need a constant pool?

Public class SimpleClass {

Public void sayHello () {

System.out.println ("hello")

}

}

Local method stack

The Java virtual machine stack is used to manage calls to Java methods, while the local method stack is used to manage calls to local methods.

Local methods are implemented in C language.

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

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