Title: In-depth analysis of the internal mechanism of the JVM: Exploring the operating principle of the Java virtual machine
Abstract: This article will deeply analyze the internal mechanism of the Java Virtual Machine (JVM) and explore its operating principle. We will discuss the structure of JVM, memory management, garbage collection, just-in-time compilation, etc., and deepen our understanding through sample codes.
text:
1. The structure of the JVM
The Java Virtual Machine (JVM) is the running environment of Java programs, which provides an isolated execution environment so that Java programs can run on different operating systems. The structure of JVM is mainly divided into three parts: class loader, runtime data area and execution engine.
- class loader
The class loader is responsible for loading Java class files into memory and generating corresponding Class objects. JVM provides three class loaders by default: startup class loader, extension class loader and application class loader. They are responsible for loading class files in different locations according to a certain hierarchical relationship.
- runtime data area
The runtime data area is the area used by the JVM to store program runtime data. It includes method area, heap, stack, native method stack and program counter. Among them, the method area is used to store the structural information of the class; the heap is used to store the object instance; the stack is used to store the local variables and method call information of the method; the local method stack is used to support the call of the local method; the program counter is used to record the current thread The address of the bytecode instruction to execute.
- execution engine
The execution engine is responsible for executing Java bytecode instructions. JVM has two execution engines: interpreter and just-in-time compiler. The interpreter interprets and executes bytecode instructions one by one, which is inefficient; while the just-in-time compiler compiles hot code (code that is executed multiple times) into local machine code to improve execution efficiency.
2. Memory management
The memory management of the JVM mainly includes two aspects of memory allocation and garbage collection.
- memory allocation
The JVM's heap is used to store object instances. The memory allocation of objects is mainly through two methods: "pointer collision" and "free list". Pointer collision means that when there is a pointer between the allocated memory and the unallocated memory in the heap as a dividing line, when the memory is allocated, the pointer moves a certain distance to the unallocated memory; the free list refers to the allocated and unallocated memory in the heap. The memory is randomly distributed, and when allocating memory, find a suitable memory block in the free list.
- garbage collection
Garbage collection is the mechanism by which the JVM automatically manages memory. The JVM determines which objects are reachable and which objects are not reachable by tracking the reference relationship of objects. Unreachable objects are reclaimed by the garbage collector, freeing memory. JVM's garbage collection algorithms include mark clearing, copying, and mark finishing.
3. Just-in-time compilation
Just-in-time compilation (JIT) is a way for the JVM to improve execution efficiency. JIT compiles hot code (code that is executed multiple times) into local machine code to replace interpreted bytecode. The JVM's just-in-time compiler has a C1 compiler and a C2 compiler.
Sample code:
The following is a simple sample code to illustrate the execution process of the JVM:
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}
In this sample code, the JVM will first load HelloWorldthe bytecode file of the class and generate the corresponding Class object. Then, when mainthe method is executed, the JVM creates a new thread and allocates a stack frame to the thread. The stack frame is used to store local variables and method call information of the method.
Inside mainthe method, the JVM executes System.out.printlnthe method, which "Hello, World!"outputs a string to the console. The JVM will execute the code one by one according to the bytecode instructions, and convert the bytecode instructions into machine code execution through the execution engine.
Finally, when mainthe method finishes executing, the JVM exits the program and frees the allocated memory.
in conclusion:
This article deeply analyzes the internal mechanism of the JVM, including the structure of the JVM, memory management, garbage collection, and just-in-time compilation. Through sample code, we can better understand the operation principle of JVM. A deep understanding of the internal mechanism of the JVM helps developers write efficient Java programs and better tune and troubleshoot problems.