JVM 第三阶段--Java内存机制(2020-02)

内存模型初识

虚拟机栈:stack Frame栈桢

程序计数器:(program Counter,PC)

本地方法栈:主要用于处理本地方法

堆(Heap):JVM最大的一块内存空间,与堆相关的是垃圾收集器.现代几乎所有的垃圾收集器都是分代收集算法,所以堆空间也基于此进行划分:新生代与老年代.Eden,From survivor,To suivivor.

方法区(Method Area):存储元信息.永久代(Permanent Generation),从JDK1.8废弃,使用元空间(meta space)

直接内存:(Direct Memory)

new关键字创建对象的三个步骤:
	1.在堆内存中创建出对象实例
	2.为对象的实例成员变量赋初值
	3.将对象的引用返回

-Xms5m -Xmx5m -XX:+HeapDumpOnOutOfMemoryError

//堆内存溢出
public class MyTest1 {

    public static void main(String[] args) {
         List<MyTest1> list = new ArrayList<>();
         for (;;){
             list.add(new MyTest1());
             //System.gc();
         }
    }
}

java.lang.OutOfMemoryError: Java heap space
Dumping heap to java_pid1045.hprof ...
Heap dump file created [9059207 bytes in 0.141 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
	at com.cy.jvm.memory.MyTest1.main(MyTest1.java:13)

//虚拟机栈溢出. -Xss160k 160k为推荐栈的最小内存
public class MyTest2 {

    private int length;

    public int getLength() {
        return length;
    }

    public void setLength(int length) {
        this.length = length;
    }

    public void test() {
        this.length++;
        test();
    }

    public static void main(String[] args) {
        MyTest2 myTest2 = new MyTest2();
        try {
            myTest2.test();
        } catch (Throwable e) {
            System.out.println(myTest2.length);
            e.printStackTrace();
        }
    }
}

772
java.lang.StackOverflowError
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:17)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	at com.cy.jvm.memory.MyTest2.test(MyTest2.java:18)
	.....

//死锁
public class MyTest3 {
    public static void main(String[] args) {
        new Thread(() -> A.method(), "Thread-A").start();
        new Thread(() -> B.method(), "Thread-B").start();
    }
}

class A {
    public static synchronized void method() {
        System.out.println("method A");
        try {
            Thread.sleep(5000);
            B.method();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

class B {
    public static synchronized void method() {
        System.out.println("method B");
        try {
            Thread.sleep(5000);
            A.method();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

method A
method B

-XX:MaxMetaspaceSize=200m
/**
 * 方法区内存溢出错误
 */
public class MyTest4 {

    public static void main(String[] args) {
        for (; ; ) {
            Enhancer enhancer = new Enhancer();
            enhancer.setSuperclass(MyTest4.class);
            enhancer.setUseCache(false);
            enhancer.setCallback(((MethodInterceptor) (obj, method, args1, proxy) -> proxy.invokeSuper(obj, args1)));
            System.out.println("hello world");
            enhancer.create();
        }
    }
}
.....
hello world
hello world
hello world
Exception in thread "main" 
Exception: java.lang.OutOfMemoryError thrown from the UncaughtExceptionHandler in thread "main"