欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页

System.gc()与Runtime.getRuntime().gc()区别

程序员文章站 2022-06-15 16:41:36
...
首先,我们需要明确一点的是,两个gc都会强制触发垃圾收集,它们只是向JVM建议,现在也许是收集垃圾的好时机。
比较
System.gc()调用起来更方便,但是会给应用带来不必要的性能问题。还可以通过参数-XX:+DisableExplicitGC.禁止显示调用gc。
Runtime.getRuntime()用来与Java运行时进行交互,调用该方法会建议JVM花费精力回收不再使用的对象。
源代码
System.gc()源代码, 来自System.java

  /**
     * Runs the garbage collector.
     * <p>
     * Calling the <code>gc</code> method suggests that the Java Virtual
     * Machine expend effort toward recycling unused objects in order to
     * make the memory they currently occupy available for quick reuse.
     * When control returns from the method call, the Java Virtual
     * Machine has made a best effort to reclaim space from all discarded
     * objects.
     * <p>
     * The call <code>System.gc()</code> is effectively equivalent to the
     * call:
     * <blockquote><pre>
     * Runtime.getRuntime().gc()
     * </pre></blockquote>
     *
     * @see     java.lang.Runtime#gc()
     */
    public static void gc() {
        Runtime.getRuntime().gc();
    }


Runtime.getRuntime().gc()源代码,来自Runtime.java


/**
     * Runs the garbage collector.
     * Calling this method suggests that the Java virtual machine expend
     * effort toward recycling unused objects in order to make the memory
     * they currently occupy available for quick reuse. When control
     * returns from the method call, the virtual machine has made
     * its best effort to recycle all discarded objects.
     * <p>
     * The name <code>gc</code> stands for "garbage
     * collector". The virtual machine performs this recycling
     * process automatically as needed, in a separate thread, even if the
     * <code>gc</code> method is not invoked explicitly.
     * <p>
     * The method {@link System#gc()} is the conventional and convenient
     * means of invoking this method.
     */
    public native void gc();


示例:



import lombok.extern.slf4j.Slf4j;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

/**
 * Created by EricYang on 2019/7/13.
 */
@Slf4j
public class GcMainDemo {
    public static void main(String[] args) {
        List<String> stringList = new ArrayList<>();
        for(int i=0;i < 100; i++) {
            stringList.add(new String("The " + i + "th string"));
        }

        //unit bytes
        long maxMemBefore = Runtime.getRuntime().freeMemory();
        //System.gc();
        Runtime.getRuntime().gc();
        long maxMemAfter = Runtime.getRuntime().freeMemory();
        log.info("System.gc(), maxMemBefore={}, maxMemAfter={}, diff={}kb", maxMemBefore, maxMemAfter, (maxMemAfter - maxMemBefore)/1024);
        for(int i=0;i < 100; i++) {
            stringList.add(new String("The " + i + "th string"));
        }
        maxMemBefore = Runtime.getRuntime().freeMemory();
        Runtime.getRuntime().gc();
        maxMemAfter = Runtime.getRuntime().freeMemory();
        log.info("Runtime.getRuntime().gc(), maxMemBefore={}, maxMemAfter={}, diff={}kb", maxMemBefore, maxMemAfter, (maxMemAfter - maxMemBefore)/1024);
        try {
            TimeUnit.SECONDS.sleep(2000);
        } catch (Exception ex) {
            log.error("sleep exception", ex);
        }
    }
}



程序运行输出(不同机器运行结果不一样)
16:39:22.616 [main] INFO com.yq.GcMainDemo - System.gc(), maxMemBefore=244672784, maxMemAfter=253615304, diff=8732kb
16:39:22.635 [main] INFO com.yq.GcMainDemo - Runtime.getRuntime().gc(), maxMemBefore=250951864, maxMemAfter=253949240, diff=2927kb
结论
两者一样。除了方法名称不同, System.gc()是静态方法, 而Runtime.getRuntime().gc()是实例方法,没有实质差别。

相关标签: java jvm