这几天都在外面跑，没时间上机调代码，晚上回来趁这机会研究下，结果如下：

bobguo@bobguo-PC:~/source/eetest$ java -version && javac hash.java && cat hash.java && time java -Xms2G -verbose:gc hash && time java -verbose:gc hash
Picked up _JAVA_OPTIONS:   -Dawt.useSystemAAFontSettings=gasp
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)
Picked up _JAVA_OPTIONS:   -Dawt.useSystemAAFontSettings=gasp
import java.util.HashMap;
import java.util.Map;

public class hash {

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        String time;
        Map<String, String> map = new HashMap<>();
        for (int i = 0; i < 1000000; i++) {
            time = String.valueOf(System.currentTimeMillis());
            map.put(i + "_" + time, time);
        }
        System.out.println(System.currentTimeMillis() - start);
    }

}
Picked up _JAVA_OPTIONS:   -Dawt.useSystemAAFontSettings=gasp
690

real    0m0.806s
user    0m0.964s
sys    0m0.104s
Picked up _JAVA_OPTIONS:   -Dawt.useSystemAAFontSettings=gasp
[GC (Allocation Failure)  31744K->16720K(121856K), 0.0235111 secs]
[GC (Allocation Failure)  48464K->35592K(153600K), 0.0363266 secs]
[GC (Allocation Failure)  99080K->96188K(160256K), 0.0982848 secs]
[Full GC (Ergonomics)  96188K->95872K(278528K), 1.0974248 secs]
[GC (Allocation Failure)  159091K->145848K(295424K), 0.1148326 secs]
[Full GC (Ergonomics)  145848K->145669K(398336K), 0.9425082 secs]
2901

real    0m3.010s
user    0m9.548s
sys    0m0.200s

这个场景特殊在哪里呢？特殊在java擅长长期运行，所以针对这种从0开始剧烈使用内存，然后直接结束的的场景，默认参数并没有很好的优化导致的。