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   #查看是否用到了G1，有'-'号表示没有用到

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

jinfo -flags PID   #查看全部

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

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

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

-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=heap.hprof