JVM内存增强之逃逸分析

//对象发生了逃逸,不会在栈上分配,有可能导致GC STW
public StringBuffer append(String s1, String s2) {
 StringBuffer sb = new StringBuffer();
 sb.append(s1);
sb.append(s2);
 return sb;
}
//对象未发生逃逸
public String append(String s1, String s2) {
 StringBuffer sb = new StringBuffer();
 sb.append(s1);
 sb.append(s2);
 return sb.toString();
}

/**
* 栈上分配测试(-XX:+DoEscapeAnalysis)
* -Xmx128m -Xms128m -XX:+DoEscapeAnalysis -XX:+PrintGC
*/
public class ObjectStackAllocationTests {
 public static void main(String[] args) throws InterruptedException {
 long start = System.currentTimeMillis();
 for (int i = 0; i < 10000000; i++) {
 alloc();
 }
 long end = System.currentTimeMillis();
 System.out.println("花费的时间为： " + (end - start) + " ms");
 // 为了方便查看堆内存中对象个数，线程 sleep
 TimeUnit.MINUTES.sleep(5);
 }
 private static void alloc() {
 byte[] data = new byte[10];//未发生逃逸
 }
}

public class SynchronizedLockTest {
 public void lock() {
 Object obj= new Object();
 synchronized(obj) {
 System.out.println(obj);
 }
}

public class ObjectScalarReplaceTests {
 public static void main(String args[]) {
 long start = System.currentTimeMillis();
 for (int i = 0; i < 10000000; i++) {
 alloc();
 }
 long end = System.currentTimeMillis();
 System.out.println("花费的时间为： " + (end - start) + " ms");
 }
 private static void alloc() {
 Point point = new Point(1,2);
 }
 static class Point {
 private int x;
 private int y;
 public Point(int x,int y){
 this.x=x;
 this.y=y;
 }
 }
//对于上面代码，假如开启了标量替换，那么 alloc 方法的内容就会变为如下形式
private static void alloc() {
     int x=10;
     int y=20;
 }