java使用system.gc()内存(堆栈)回收

╰半夏微凉° 2024-03-17 19:20 41阅读 0赞

### 1：window窗口可以使用cmd命令jconsole，弹出 ###

![在这里插入图片描述][db0b90e87fb445259e8cbecc8f8ad6f3.jpeg_pic_center]

> 注意：不可以使用debug模式，可以使用代码现场睡眠模式测试，启动main方法后，在本地进程选择相应的执行类

public class Main2 {
        
        @SneakyThrows
        public static void main(String[] args) {
        
            if (true) {
        
                Thread.sleep(5000);
                byte[] placeHolder = new byte[64 * 1024 * 1024];
                System.out.println(placeHolder.length / 1024);
                Thread.sleep(5000);
            }
            System.gc();
            Thread.sleep(2000);
        }
    }

### 注意：在执行System.gc()没存没有进行回收，因为有新建对象在，内存在第12秒后全部回收，是因为整个main方法执行结束了 ###

![在这里插入图片描述][c420a238984d48e29967e632675a1075.jpeg_pic_center]

public class Main2 {
        
        @SneakyThrows
        public static void main(String[] args) {
        
            if (true) {
        
                Thread.sleep(5000);
                byte[] placeHolder = new byte[64 * 1024 * 1024];
                System.out.println(placeHolder.length / 1024);
                Thread.sleep(5000);
                placeHolder=null;
            }
            System.gc();
            Thread.sleep(2000);
        }

### 注意：内存在第10秒后就差不多全部回收，是因为placeHolder=null，剩下的就是main方法全部结束后回收了 ###

![在这里插入图片描述][234473552a94470a9a5eb3368d42e9d5.jpeg_pic_center]

### 2：使用java命令启动，如果类里面有包含包路径，就切换到类里面包路径的根目录执行，如下 ###

package org.jeecg.modules.system.service.impl;
    
    import lombok.SneakyThrows;
    
    /**
     * @Author xu
     * @create 2023/7/13 01
     */
    public class Main2 {
        
        @SneakyThrows
        public static void main(String[] args) {
        
            if (true) {
        
                Thread.sleep(5000);
                byte[] placeHolder = new byte[64 * 1024 * 1024];
                System.out.println(placeHolder.length / 1024);
                Thread.sleep(5000);
                placeHolder=null;
            }
            System.gc();
            Thread.sleep(2000);
        }
    }

### 编译Main2.java为Main2.class文件 ###

G:\hdxProject\java\org\jeecg\modules\system\service\impl>javac Main2.java
    G:\hdxProject\java>

### 启动Main2类 ###

G:\hdxProject\java>java org/jeecg/modules/system/service/impl/Main2
    65536
    
    G:\hdxProject\java>

### 启动Main2类并打印内存回收信息 ###

G:\hdxProject\java>java -XX:+PrintGC org/jeecg/modules/system/service/impl/Main2
    65536
    [GC (System.gc())  69447K->66392K(373248K), 0.0203310 secs]
    [Full GC (System.gc())  66392K->66202K(373248K), 0.0147270 secs]
    
    G:\hdxProject\java>

### 启动Main2类并打印内存回收详细信息 ###

G:\hdxProject\java>java -XX:+PrintGCDetails org/jeecg/modules/system/service/impl/Main2
    65536
    [GC (System.gc()) [PSYoungGen: 69447K->784K(113664K)] 69447K->66328K(373248K), 0.0200134 secs] [Times: user=0.00 sys=0.02, real=0.02 secs]
    [Full GC (System.gc()) [PSYoungGen: 784K->0K(113664K)] [ParOldGen: 65544K->66202K(259584K)] 66328K->66202K(373248K), [Metaspace: 2735K->2735K(1056768K)], 0.0047035 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
    Heap
     PSYoungGen      total 113664K, used 978K [0x0000000741980000, 0x0000000749800000, 0x00000007c0000000)
      eden space 97792K, 1% used [0x0000000741980000,0x0000000741a74920,0x0000000747900000)
      from space 15872K, 0% used [0x0000000747900000,0x0000000747900000,0x0000000748880000)
      to   space 15872K, 0% used [0x0000000748880000,0x0000000748880000,0x0000000749800000)
     ParOldGen       total 259584K, used 66202K [0x0000000644c00000, 0x0000000654980000, 0x0000000741980000)
      object space 259584K, 25% used [0x0000000644c00000,0x0000000648ca6970,0x0000000654980000)
     Metaspace       used 2741K, capacity 4486K, committed 4864K, reserved 1056768K
      class space    used 305K, capacity 386K, committed 512K, reserved 1048576K
    
    G:\hdxProject\java>

### 3：设置idea执行System.gc()打印内存回收信息 ###

![在这里插入图片描述][0771833387c840dfaaeccc3b407e422b.jpeg_pic_center]  
![在这里插入图片描述][fd72aeeefce749bc8e0c42f88d4688c2.jpeg_pic_center]

### 设置idea执行System.gc()打印内存详细回收信息 ###

![在这里插入图片描述][760c7433a05c4678ae88d11a702707cd.jpeg_pic_center]

[db0b90e87fb445259e8cbecc8f8ad6f3.jpeg_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/a77d8930f1ff49d2b073f7d78df1bed5.png
[c420a238984d48e29967e632675a1075.jpeg_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/802780ef02ed40fd9c665667b8396700.png
[234473552a94470a9a5eb3368d42e9d5.jpeg_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/a491baa7cb26438992d78f9200f4dac4.png
[0771833387c840dfaaeccc3b407e422b.jpeg_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/31cd6f8d75e84289855ff7f922892219.png
[fd72aeeefce749bc8e0c42f88d4688c2.jpeg_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/be22c3120c0048e8b8ed25dfe8db3656.png
[760c7433a05c4678ae88d11a702707cd.jpeg_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/55fc08382a8a4c29a496908dcdd5baec.png