JVM學習之 Java記憶體結構

 二进制字节码			JVM指令					Java源代码 0: getstatic     #20                 // PrintStream out = System.out; 
 3: astore_1                          // - 
 4: aload_1                           // out.println(1); 
 5: iconst_1                          // - 
 6: invokevirtual #26                 // - 
 9: aload_1                           // out.println(2); 
 10: iconst_2                          // -
 11: invokevirtual #26                 // -
 14: aload_1                           // out.println(3); 
 15: iconst_3                          // -
 16: invokevirtual #26                 // -
 19: aload_1                           // out.println(4); 
 20: iconst_4                          // -
 21: invokevirtual #26                 // -
 24: aload_1                           // out.println(5); 
 25: iconst_5                          // -
 26: invokevirtual #26                 // -
 29: return

public class teststacks {
	public static void main(String[] args) throws InterruptedException{
		method1();
	}
	public static void method1(){
		method2(1,2);
	}
	public static int method2(int a,int b){
		int c=a+b;
		return c;
	}}

	//多个线程同时执行该方法，会不会造成x值混乱呢？
	//不会，因为x是方法内的局部变量，是线程私有的，互不干扰
	static void m1(){
		int x=0;
		for(int i=0;i<p>但是如果我们把变量的类型改为static,此时就大不一样了，x是静态变量，线程1和线程2同时拥有同一个x，static变量针对多个线程是一个共享的，不加安全保护的话，就会出现线程安全问题。</p><pre class="brush:php;toolbar:false">	static void m1(){
		static int x=0;
		for(int i=0;i<p>我们再看几个方法</p><pre class="brush:php;toolbar:false">public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        sb.append(4);
        sb.append(5);
        sb.append(6);
        new Thread(()->{
            m2(sb);
        }).start();
    }
    public static void m1() {
        StringBuilder sb = new StringBuilder();
        sb.append(1);
        sb.append(2);
        sb.append(3);
        System.out.println(sb.toString());
    }
    public static void m2(StringBuilder sb) {
        sb.append(1);
        sb.append(2);
        sb.append(3);
        System.out.println(sb.toString());
    }
    public static StringBuilder m3() {
        StringBuilder sb = new StringBuilder();
        sb.append(1);
        sb.append(2);
        sb.append(3);
        return sb;
    }

public class Demo1_2 {
	private static int count;
    public static void main(String[] args) {
        try {
            method1();
        } catch (Throwable e) {
            e.printStackTrace();
            System.out.println(count);
        }
    }
    private static void method1() {
        count++;
        method1();
    }}

public native int hashCode();

public static void main(String[] args) {
        int i = 0;
        try {
            List<string> list = new ArrayList();
            String a = "hello";
            while (true) {
                list.add(a); // hello, hellohello, hellohellohellohello ...
                a = a + a;  // hellohellohellohello
                i++;
            }
        } catch (Throwable e) {
            e.printStackTrace();
            System.out.println(i);
        }}</string>

/**
 * 演示元空间内存溢出 java.lang.OutOfMemoryError: Metaspace
 * -XX:MaxMetaspaceSize=8m
 */public class Demo1_8 extends ClassLoader { // 可以用来加载类的二进制字节码
    public static void main(String[] args) {
        int j = 0;
        try {
            Demo1_8 test = new Demo1_8();
            
            for (int i = 0; i <p><strong>jdk1.8以后， 默认情况下，方法区用的是系统内存，所以加大还是不会导致内存溢出，循环很多次都运行成功。</strong><br><strong>当设置了-XX:MaxMetaspaceSize=8m，到了5411次就溢出了。报的是java.lang.OutOfMemoryError: Metaspace错误</strong></p><p>而1.8以前永久代溢出报的错误是java.lang.OutOfMemoryError:PermGen space</p><p><strong>6.4.常量池</strong></p><p><img src="https://img.php.cn/upload/article/000/000/052/b93dd11973d3f8094228c146b3131c12-17.png" alt="JVM學習之 Java記憶體結構"></p><p><strong>常量池，就是一张表，虚拟机指令根据这站常量表找到要执行的类名、方法名、参数类型、字面量信息(如字符串常量、true和false)</strong>。<br><em><em>运行时常量池，常量池是</em>.class文件中的，当该类被加载，它的常量池信息就会放入运行时常量池，并把里面的符号地址变为真实地址</em>*。</p><pre class="brush:php;toolbar:false">public class HelloWorld {
	public static void main(String[] args) {
		System.out.println("hello,world");
	}}

F:\IDEA\projects\jvm>javap -v F:\IDEA\projects\jvm\out\production\untitled\HelloWorld.class

Classfile /F:/IDEA/projects/jvm/out/production/untitled/HelloWorld.class
  Last modified 2021-1-30; size 533 bytes
  MD5 checksum 82d075eb7217b4d23706f6cfbd44f8f1
  Compiled from "HelloWorld.java"public class HelloWorld
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:
   #1 = Methodref          #6.#20         // java/lang/Object."<init>":()V
   #2 = Fieldref           #21.#22        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #23            // hello,world
   #4 = Methodref          #24.#25        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #26            // HelloWorld
   #6 = Class              #27            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               LocalVariableTable
  #12 = Utf8               this
  #13 = Utf8               LHelloWorld;
  #14 = Utf8               main
  #15 = Utf8               ([Ljava/lang/String;)V
  #16 = Utf8               args
  #17 = Utf8               [Ljava/lang/String;
  #18 = Utf8               SourceFile
  #19 = Utf8               HelloWorld.java
  #20 = NameAndType        #7:#8          // "<init>":()V
  #21 = Class              #28            // java/lang/System
  #22 = NameAndType        #29:#30        // out:Ljava/io/PrintStream;
  #23 = Utf8               hello,world
  #24 = Class              #31            // java/io/PrintStream
  #25 = NameAndType        #32:#33        // println:(Ljava/lang/String;)V
  #26 = Utf8               HelloWorld
  #27 = Utf8               java/lang/Object
  #28 = Utf8               java/lang/System
  #29 = Utf8               out
  #30 = Utf8               Ljava/io/PrintStream;
  #31 = Utf8               java/io/PrintStream
  #32 = Utf8               println
  #33 = Utf8               (Ljava/lang/String;)V</init></init></init>

{
  public HelloWorld();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature            0       5     0  this   LHelloWorld;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String hello,world
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 3: 0
        line 4: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature            0       9     0  args   [Ljava/lang/String;}</init>