-version
-help
-server
-cp

-Xint     解释执行
-Xcomp    第一次使用就编译成本地代码
-Xmixed   混合模式，JVM自己来决定

a.Boolean类型
格式：-XX:[+-]<name>            +或-表示启用或者禁用name属性
比如：-XX:+UseConcMarkSweepGC   表示启用CMS类型的垃圾回收器
 -XX:+UseG1GC              表示启用G1类型的垃圾回收器
b.非Boolean类型  
格式：-XX<name>=<value>表示name属性的值是value
比如：-XX:MaxGCPauseMillis=500

-Xms1000M等价于-XX:InitialHeapSize=1000M
-Xmx1000M等价于-XX:MaxHeapSize=1000M
-Xss100等价于-XX:ThreadStackSize=100

1Byte(字节)=8bit(位)
1KB=1024Byte(字节)
1MB=1024KB
1GB=1024MB
1TB=1024GB

(1)设置堆内存大小和参数打印
-Xmx100M -Xms100M -XX:+PrintFlagsFinal
(2)查询+PrintFlagsFinal的值
:=true
(3)查询堆内存大小MaxHeapSize
:= 104857600
(4)换算
104857600(Byte)/1024=102400(KB)
102400(KB)/1024=100(MB)
(5)结论
104857600是字节单位

The jps command lists the instrumented Java HotSpot VMs on the target system. The command is limited to reporting information on JVMs for which it has the access permissions.

The jinfo command prints Java configuration information for a specified Java process or core file or a remote debug server. The configuration information includes Java system properties and Java Virtual Machine (JVM) command-line flags.

jinfo -flag MaxHeapSize PID 
jinfo -flag UseG1GC PID

jinfo -flag [+|-] PID
jinfo -flag <name>=<value> PID

jinfo -flags PID

The jstat command displays performance statistics for an instrumented Java HotSpot VM. The target JVM is identified by its virtual machine identifier, or vmid option.

jstat -class PID 1000 10   查看某个java进程的类装载信息，每1000毫秒输出一次，共输出10次

jstat -gc PID 1000 10

The jstack command prints Java stack traces of Java threads for a specified Java process, core file, or remote debug server.

jstack PID

//运行主类
public class DeadLockDemo
{
    public static void main(String[] args)
    {
        DeadLock d1=new DeadLock(true);
        DeadLock d2=new DeadLock(false);
        Thread t1=new Thread(d1);
        Thread t2=new Thread(d2);
        t1.start();
        t2.start();
    }
}
//定义锁对象
class MyLock{
    public static Object obj1=new Object();
    public static Object obj2=new Object();
}
//死锁代码
class DeadLock implements Runnable{
    private boolean flag;
    DeadLock(boolean flag){
        this.flag=flag;
    }
    public void run() {
        if(flag) {
            while(true) {
                synchronized(MyLock.obj1) {
                    System.out.println(Thread.currentThread().getName()+"----if获得obj1锁");
                    synchronized(MyLock.obj2) {
                        System.out.println(Thread.currentThread().getName()+"----if获得obj2锁");
                    }
                }
            }
        }
        else {
            while(true){
                synchronized(MyLock.obj2) {
                    System.out.println(Thread.currentThread().getName()+"----否则获得obj2锁");
                    synchronized(MyLock.obj1) {
                        System.out.println(Thread.currentThread().getName()+"----否则获得obj1锁");
                    }
                }
            }
        }
    }
}

The jmap command prints shared object memory maps or heap memory details of a specified process, core file, or remote debug server.

jmap -heap PID

jinfo -flag UsePSAdaptiveSurvivorSizePolicy 35352
-XX:SurvivorRatio=8

jmap -dump:format=b,file=heap.hprof PID

设置堆内存大小: -Xms20M -Xmx20M
启动，然后访问localhost:9090/heap，使得堆内存溢出

java -jar -Xms1000M -Xmx1000M -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=jvm.hprof  jvm-case-0.0.1-SNAPSHOT.jar

top
top -Hp PID

jstack PID
java -jar arthas.jar   --->   thread

jmap -heap PID
java -jar arthas.jar   --->   dashboard

jmap -histo:live PID | more
获取到jvm.hprof文件，上传到指定的工具分析，比如heaphero.io

（1）50%以上的堆被存活对象占用
（2）对象分配和晋升的速度变化非常大
（3）垃圾回收时间比较长

Throughput       Min Pause       Max Pause      Avg Pause       GC count
  99.16%         0.00016s         0.0137s        0.00559s          12

-XX:MetaspaceSize=100M
-Xms300M
-Xmx300M

Throughput       Min Pause       Max Pause      Avg Pause       GC count
  98.89%          0.00021s        0.01531s       0.00538s           12

-XX:MaxGCPauseMillis=200    设置最大GC停顿时间指标

Throughput       Min Pause       Max Pause      Avg Pause       GC count
  98.96%          0.00015s        0.01737s       0.00574s          12

-XX:InitiatingHeapOccupancyPercent=45 
G1用它来触发并发GC周期,基于整个堆的使用率,而不只是某一代内存的使用比例。值为 0 则表示“一直执行GC循环)'. 默认值为 45 (例如, 全部的 45% 或者使用了45%).

Throughput       Min Pause       Max Pause      Avg Pause       GC count
  98.11%          0.00406s        0.00532s       0.00469s          12

有时候会发现系统刚刚启动的时候，就会发生一次Full GC，但是老年代空间比较充足，一般是由Metaspace区域引起的。可以通过MetaspaceSize适当增加其大家，比如256M。

Throughput       Min Pause       Max Pause      Avg Pause       GC count
  99.16%         0.00016s         0.0137s        0.00559s          12