Java Learning Jvm Garbage Collector (Basics)

This article will bring you an introduction to the Jvm garbage collector (basics) for java learning. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1: Overview

In this article "Jvm Runtime Data Area" introduces various parts of the Java memory runtime area, including the program counter, virtual machine stack, and local method stack. The three areas survive as the thread survives. Memory allocation and deallocation are deterministic. The memory is naturally recycled as the thread ends, so there is no need to consider garbage collection. The Java heap and method area are different. They are shared by each thread, and memory allocation and recycling are dynamic. Therefore, the garbage collector focuses on this part of memory.

Next we will discuss how the Jvm reclaims this part of memory. The first thing the garbage collector does before recycling is to determine which objects are still alive and which are dead. Two basic recycling algorithms are introduced below.

1.1 Reference counting algorithm

Add a reference counter to the object. Whenever there is a reference to it, the counter will be 1, and when the reference expires, the counter will be -1. As long as the counter is equal to 0, the object cannot be used anymore.

This algorithm is a good choice in most cases, and there are also some famous application cases. But it is not used in the Java virtual machine.

Advantages: Simple implementation and high judgment efficiency.

Disadvantages: It is difficult to solve the problem of circular references between objects. For example, the following example

Object a = new Object(); 
Object b = new Object(); 
a=b; 
b=a; 
a=b=null; //这样就导致gc无法回收他们。　　

1.2 Reachability analysis algorithm

uses a series of objects called "GC Roots" as the starting point, and searches downward from these nodes to search for the path. The path passed is called a reference chain. When an object does not use any reference chain to GC Roots, it means that the object is unavailable.

In mainstream implementations of mainstream commercial programming languages ​​(Java, C#, etc.), reachability analysis is used to determine whether an object is alive.

Through the following figure, you can clearly feel the connection between gc root and object display. The objects in the gray area shown are alive, and Object5/6/7 are all recyclable objects

In Java language, the objects that can be used as GC Roots include the following types

• Objects referenced in the virtual machine stack (local variable table in the stack frame)

• Objects referenced by static variables in the method area

• Objects referenced by constants in the method area

• Objects referenced by JNI in the local method stack (generally speaking Native methods)

Advantages: More accurate and rigorous, can analyze the mutual reference of cyclic data structures;

Disadvantages: The implementation is more complex and requires Analyzing a large amount of data consumes a lot of time, and the analysis process requires GC pauses (reference relationships cannot change), that is, stopping all Java execution threads (called "Stop The World", which is a key issue of garbage collection).

2: Reference

After jdk1.2, Java has expanded the concept of reference, which is generally divided into four categories: strong reference, soft reference, weak reference, and virtual reference. These four The citation intensity gradually weakens.

• Strong references: Refers to references that are common in the code, such as Object obj = new Object(); Only strong references can still exists, the GC will never collect the referenced object.

• Soft reference: refers to some objects that are useful but not necessary. It will not be garbage collected until there is not enough memory space (before OutOfMemoryError is thrown). Use the SoftReference class to implement soft references

• ##Weak reference: Used to describe non-essential objects. Such objects will be recycled when the garbage collector works. Use the WeakReference class to implement weak references.

• ##Virtual reference: Whether an object has a virtual reference, It will not affect its survival time at all. The only purpose is to receive a system notification when the object is recycled. Use the PhantomRenference class to implement

2.1 Determine an object Life or Death

To declare an object dead, it must be marked at least twice.

If the object does not find a reference chain connected to GC Roots after the reachability analysis algorithm, it will be marked for the first time. Mark and filter.

: Determine whether it is necessary for this object to execute the finalize() method.

Filtering results: When the object does not cover the finalize() method, or the finalize() method has been executed by the JVM, it is determined to be a recyclable object. If it is necessary for the object to execute the finalize() method, it will be placed in the F-Queue queue. This method will be triggered later in the JVM automatically created, low-priority Finalizer thread (possibly multiple threads); The object is marked twice.

The above is the detailed content of Java Learning Jvm Garbage Collector (Basics). For more information, please follow other related articles on the PHP Chinese website!