JVM逃逸分析
JVM有栈、堆、方法区、本地栈等组成
栈：每个方法被执行的时候都会同时创建一个栈帧用于存储局部变量表、操作栈、动态连接、方法出口等信息。每个方法被调用直至执行完成的过程，就对应着一个栈帧在虚拟机栈中从入栈到出栈的过程。
堆：当实例化对象时，会把对象分配到堆中，然后把指向该堆的引用压入栈中。
逃逸：当一个对象的指针被多个方法或线程引用时，我们称这个指针发生了逃逸，一般情况返回对象、对全局变量的肤质一般都会发生逃逸。
逃逸分析：用来分析这种逃逸现象的方法称为逃逸分析
逃逸分析优化-栈上分配：栈上分配的意思是方法内局部变量（未发生逃逸）生成的实例在栈上分配，不用在堆中分配，分配完成后，继续在调用栈内执行，最后线程结束，栈空间被回收，局部变量对象也被回收。
代码测试：
import java.lang.management.ManagementFactory;
import java.util.List;
/**
* 逃逸分析优化-栈上分配
* 栈上分配,意思是方法内局部变量（未发生逃逸）生成的实例在栈上分配，不用在堆中分配，分配完成后，继续在调用栈内执行，最后线程结束，栈空间被回收，局部变量对象也被回收。
* 一般生成的实例都是放在堆中的,然后把实例的指针或引用压入栈中。
*虚拟机参数设置如下，表示做了逃逸分析 消耗时间在10毫秒以下
* -server -Xmx10M -Xms10M
-XX:+DoEscapeAnalysis -XX:+PrintGC
*
*虚拟机参数设置如下，表示没有做逃逸分析 消耗时间在1000毫秒以上
* -server -Xmx10m -Xms10m
-XX: -DoEscapeAnalysis -XX:+PrintGC
* @author 734621
*
*/
public class OnStack{
public static void alloc(){
byte[] b=new byte[2];
b[0]=1;
}
public static void main(String [] args){
long b=System.currentTimeMillis();
for(int i=0;i<100000000;i++){
alloc();
}
long e=System.currentTimeMillis();
System.out.println("消耗时间为:" + (e - b));
List<String> paramters = ManagementFactory.getRuntimeMXBean().getInputArguments();
for(String p : paramters){
System.out.println(p);
}
}
}
打印结果：
加逃逸分析的结果
[GC (Allocation Failure) 2816K->484K(9984K), 0.0013117 secs]
消耗时间为:7
-Xmx10m
-Xms10m
-XX:+DoEscapeAnalysis
-XX:+PrintGC
-Dfile.encoding=GBK
没有加逃逸分析的结果如下：
[GC (Allocation Failure) 3320K->504K(9984K), 0.0003174 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002524 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002618 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001474 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002843 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002922 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002190 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003259 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002738 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001946 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0028288 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0004558 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0106963 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002351 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001471 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002494 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002187 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002732 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001847 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002922 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002773 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002999 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002017 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001205 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002905 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002952 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002676 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001647 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001319 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001319 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002744 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002931 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001762 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001480 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002884 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001659 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002990 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003104 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0004854 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002767 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002489 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001392 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002272 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002641 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002826 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003180 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002714 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002166 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002749 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003793 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002362 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002714 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002764 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002981 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002723 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002324 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002647 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002591 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002875 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001820 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002729 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002931 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002251 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002676 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003130 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002143 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002881 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002603 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002556 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003966 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002749 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002949 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0006170 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0249173 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002620 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001914 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0028737 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0006000 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003945 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002313 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002881 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002544 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002140 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001773 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002650 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002943 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002201 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003274 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001381 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002442 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003031 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003465 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001577 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003189 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002609 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002348 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002216 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0009793 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001263 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002843 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002588 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002175 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0025132 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002579 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002491 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0005171 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003189 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002497 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002471 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001747 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0104052 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002840 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0009805 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0105928 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002620 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0038738 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002116 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002157 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0110542 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0104225 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002899 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002474 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001946 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003013 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002776 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003992 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003031 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002597 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003230 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003916 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002820 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002509 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002650 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002442 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0055639 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0109589 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0009693 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0020453 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0037897 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0109237 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002914 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002685 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0109944 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002720 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002644 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002638 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002471 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003101 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002518 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002858 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002752 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003453 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002609 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0108493 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002298 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0066162 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003078 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002615 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002673 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002532 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002659 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001762 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002937 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002234 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0009092 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002987 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002149 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002568 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002362 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002521 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001650 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003233 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002360 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001700 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002248 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0004145 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0008594 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0029256 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003189 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003497 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003242 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002116 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002837 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002931 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0002553 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0003579 secs] [GC (Allocation Failure) 3320K->504K(9984K), 0.0001850 secs]
消耗时间为:1150
-Xmx10m
-Xms10m
-XX:-DoEscapeAnalysis
-XX:+PrintGC
-Dfile.encoding=GBK
以上测试可以看出，栈上分配可以明显提高效率
参照来源：https://blog.csdn.net/simba_1986/article/details/54599952
原文地址：https://www.cnblogs.com/xianfengzhike/p/9128064.html
时间： 2024-07-31 19:35:00