JVM如何打印GC时间？

Question

My java code:

package com.v2ex;

public class Test {

    private static final int MB = 1024 * 1024;

    public static void main(String[] args) {
        byte[] bytes1, bytes2, bytes3, bytes4;
        bytes1 = new byte[2 * MB];
        bytes2 = new byte[2 * MB];
        bytes3 = new byte[2 * MB];
        bytes4 = new byte[4 * MB];
    }
}

java -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 com/v2ex/Test:

Heap
 PSYoungGen      total 9216K, used 6799K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 83% used [0x00000000ff600000,0x00000000ffca3f28,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 10240K, used 4096K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 40% used [0x00000000fec00000,0x00000000ff000010,0x00000000ff600000)
 PSPermGen       total 21504K, used 2751K [0x00000000f4600000, 0x00000000f5b00000, 0x00000000fec00000)
  object space 21504K, 12% used [0x00000000f4600000,0x00000000f48afc08,0x00000000f5b00000)

I don't how much time GC costs.

Answer 1

The reason you do not see any GC time is because there is no GC happening when running your program. If you add a manual call for a GC like in the code below you will get more output with the time the GC took.

private static final int MB = 1024 * 1024;

public static void main(String[] args) {
    byte[] bytes1, bytes2, bytes3, bytes4;
    bytes1 = new byte[2 * MB];
    bytes2 = new byte[2 * MB];
    bytes3 = new byte[2 * MB];
    bytes4 = new byte[4 * MB];
    System.gc();
}

Suddenly you also see these lines:

[GC (System.gc()) --[PSYoungGen: 6979K->6979K(9216K)] 11075K->15179K(19456K), 0.0063629 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] [Full GC (System.gc()) [PSYoungGen: 6979K->2340K(9216K)] [ParOldGen: 8200K->8193K(10240K)] 15179K->10533K(19456K), [Metaspace: 2514K->2514K(1056768K)], 0.0049945 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

Edit: The reason there is no GC happening is because your program finishes so quickly and consumes so little memory that there is no GC necessary.