JVM面试核心点

 一、JDK体系

二、JVM体系 

 

三、JVM内存模型

public class Math {

    public static final int data = 666;

    public static UserEntity user = new UserEntity();

    public int compute() { // 一个方法对应一块栈帧内存区域
        int a = 1;
        int b = 2;
        int c = (a+b)*10;

        return c;
    }

    public static void main(String[] args) {
        Math math = new Math();
        math.compute();
        // 底层会调用start0(),是一个本地方法(C++实现的private native void start0())
        new Thread().start(); 
        System.out.println("--------end--------");
    }

}

对代码进行反汇编：javap -c Math.class > math.txt

Compiled from "Math.java"
public class com.ww.sso.controller.Math {
  public static final int data;

  public static com.ww.sso.entity.UserEntity user;

  public com.ww.sso.controller.Math();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."":()V
       4: return

  public int compute();
    Code:
       0: iconst_1                          // JVM指令码
       1: istore_1
       2: iconst_2
       3: istore_2
       4: iload_1
       5: iload_2
       6: iadd
       7: bipush        10
       9: imul
      10: istore_3
      11: iload_3
      12: ireturn

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class com/ww/sso/controller/Math
       3: dup
       4: invokespecial #3                  // Method "":()V
       7: astore_1
       8: aload_1
       9: invokevirtual #4                  // Method compute:()I
      12: pop
      13: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
      16: ldc           #6                  // String --------end--------
      18: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      21: return

  static {};
    Code:
       0: new           #8                  // class com/ww/sso/entity/UserEntity
       3: dup
       4: invokespecial #9                  // Method com/ww/sso/entity/UserEntity."":()V
       7: putstatic     #10                 // Field user:Lcom/ww/sso/entity/UserEntity;
      10: return
}

public class HeapTest {
    
    byte[] a = new byte[1024*100]; // 100KB
    
    public static void main(String args) throws InterruptedException {
        List heapTestList = new ArrayList<>();
        while (true) {
            heapTestList.add(new HeapTest());
            Thread.sleep(5);
        }
    }
    
}

阿里巴巴Arthashttps://alibaba.github.io/arthas

public class ArthasTest {
    
    private static HashSet hashSet = new HashSet();
    
    private static void main(String args) {
        // 模拟CPU过高
        cpuHigh();
        // 模拟线程死锁
        deadThread();
        // 不断的想hashSet集合添加数据
        addHashSetThread();
    }

    private static void addHashSetThread() {
        new Thread(() -> {
            // 初始化常量
            int cnt = 0;
            while (true) {
                try {
                    hashSet.add("cnt"+cnt);
                    Thread.sleep(10000);
                    cnt++;
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }

    private static void deadThread() {
        new Thread(() -> {
            while (true) {
                
            }
        }).start();
    }

    private static void cpuHigh() {
        Object objA = new Object();
        Object objB = new Object();
        Thread threadA = new Thread(() -> {
            synchronized (objA) {
                System.out.println(Thread.currentThread() + "get objA");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread() + "waiting get objB");
                synchronized (objB) {
                    System.out.println(Thread.currentThread() + "get objB");
                }
            }
            
        });

        Thread threadB = new Thread(() -> {
            synchronized (objB) {
                System.out.println(Thread.currentThread() + "get objB");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread() + "waiting get objA");
                synchronized (objA) {
                    System.out.println(Thread.currentThread() + "get objA");
                }
            }
        });
        
        threadA.start();
        threadB.start();
    }

}

jad xxx.xxx.ArthasTest 命令反编译线上代码。

ognl 命令可以查看线上系统变量的值，甚至可以修改变量的值。 

四、JVM优化

可达性分析算法

将“GC Roots”对象作为起点，从这些节点向下开始向下搜索引用对象，找到的对象都标记为非垃圾对象，其余未标记的对象都是垃圾对象。 

GC Roots根节点：线程的本地变量、静态变量、本地方法栈的变量等等。

 

 案例分析

JVM参数设置：java -Xms3G -Xmx3G -Xss1M -XX:MetaspaceSize=512M

线程运行每秒产生60M对象，运行14s左右，eden区就满了，进行一次minor gc，触发对象动态年龄判断。

对象动态年龄判断

当前对象的survivor区域里（其中一块区域，放对象的那块s区），一批对象的总大小大于这块survivor区域内存大小的50%（-XX:TargetSurvivor可以指定），那么此时大于等于这批对象年龄最大值的对象，就可以直接进入老年代。例如survivor区域里现有一批对象，年龄1+年龄2+年龄n的多个对象总和超过survivor区域的50%，此时就把年龄n（含）以上的对象都放入老年代。这个规则希望那些可能是长期存活的对象尽早进入老年代。对象动态年龄判断机制一般在minor gc后出发。

问：能否对JVM调优，让其几乎不进行full gc ？ 

JVM参数设置：

java -Xms3G -Xmx3G -Xmn2G -Xss1M -XX:MetaspaceSize=512M -XX:MaxMetaspaceSize=512M