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

我不是女神ヾ 2021-12-10 10:04 233阅读 0赞

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。