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Use of unsafe class

2022-07-03 18:03:00 【Spring comes again】

Unsafe The use of the class

Unsafe It can be used to directly access the system memory resources and manage them independently , In the promotion Java Operational efficiency 、 enhance Java The language bottom level operation ability aspect has played the very big role —— It can be said that ,Unsafe Class is Java The back door left in , Provides some low-level operations , Such as direct memory access 、 Thread scheduling, etc .

Unsafe Do not belong to Java standard . The official does not recommend the use of Unsafe, And from JDK 9 Start , Official start Unsafe！
relevant Issue：https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6852936

therefore ,Unsafe Class for project practice , It doesn't mean much . However, at present, there are many easy-to-use class libraries in the industry Unsafe class , for example
java.util.concurrent.atomic A bunch of classes in the bag 、Netty、Hadoop、Kafka etc. . So it's good to know .

Different JDK In the version ,Unsafe Classes are also different , for example ：

stay JDK 8 Belong to sun.misc It's a bag ;
stay JDK 11 Belong to sun.misc Package or jdk.internal.misc Next , among jdk.internal.misc Under the Unsafe Class is more powerful .（ Should be from JDK 9 At the beginning , The author did not test it personally ）

Quick start

import sun.misc.Unsafe;
import java.lang.reflect.Field;

public class DirectMemoryTest1 {
    private static final int _1MB = 1024 * 1024;

    public static void main(String[] args) throws IllegalAccessException {
        // Get by reflection Unsafe Class and allocate direct memory through it 
        Field unsafeField = Unsafe.class.getDeclaredFields()[0];
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe) unsafeField.get(null);
        int i = 0;
        while (true) {
            unsafe.allocateMemory(_1MB);
            System.out.println(++i);
        }
    }
}

Unsafe The use of the class

See ：

JDK 11 How to use Unsafe class

TIPS
Again , In actual projects , Unless it is necessary , Try not to use Unsafe class , The official does not recommend the use of
from JDK 10 Start ,Unsafe Some functions of class have been VarHandle replace , Suggest using VarHandle

Create a JDK 11 Project

Create under the project source code path module-info.java , The contents are as follows ：

module unsafe {
      
  requires jdk.unsupported;
}

After creation , The code structure is as follows ：

|____src
| |____main
| | |____java
| | | |____module-info.java
| | | |____com
| | | | |____example
| | | | | |____demo
| | | | | | |____UnsafePlayer.java
| | | | | | |____DirectMemoryTest1.java

Test code ：

import sun.misc.Unsafe;
import java.lang.reflect.Field;

public class DirectMemoryTest1 {
      
    private static final int _1MB = 1024 * 1024;

    public static void main(String[] args) throws IllegalAccessException {
      
        // Get by reflection Unsafe Class and allocate direct memory through it 
        Field unsafeField = Unsafe.class.getDeclaredFields()[0];
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe) unsafeField.get(null);
        int i = 0;
        while (true) {
      
            unsafe.allocateMemory(_1MB);
            System.out.println(++i);
        }
  }
}