第六章 方法区
文章目录
- 前言
- 一、 栈、堆、方法区的交互关系
- 二、 方法区的理解
-
- 1、方法区在哪里
- 2、方法区的演变过程
- 三、 设置方法区的大小与 OOM
-
- 1、设置方法区内存的大小
- 2、使用CGLib 让方法区OOM
- 3、如何解决 OOM
- 4、方法区的内部结构
- 5、non-final 的类变量
- 四、 运行时常量池
-
- 1、常量池
- 2、运行时常量池
- 3、方法区使用举例
- 4、方法区的演进细节
- 5、永久代为什么要被元空间替换
- 6、测试对象分别存在哪些地方
- 7、方法区的垃圾回收
- 五、 总结
前言
一、 栈、堆、方法区的交互关系
二、 方法区的理解
1、方法区在哪里
2、方法区的演变过程
三、 设置方法区的大小与 OOM
1、设置方法区内存的大小
package com.atguigu.java;
/**
* 测试设置方法区大小参数的默认值
*
* jdk7及以前：
* -XX:PermSize=100m -XX:MaxPermSize=100m
*
* jdk8及以后：
* -XX:MetaspaceSize=100m -XX:MaxMetaspaceSize=100m
* @author shkstart [email protected]
* @create 2020 12:16
*/
public class MethodAreaDemo {
public static void main(String[] args) {
System.out.println("start...");
// try {
// Thread.sleep(1000000);
// } catch (InterruptedException e) {
// e.printStackTrace();
// }
System.out.println("end...");
}
}
2、使用CGLib 让方法区OOM
package com.atguigu.java;
import com.sun.xml.internal.ws.org.objectweb.asm.ClassWriter;
import jdk.internal.org.objectweb.asm.Opcodes;
/**
* jdk6/7中：
* -XX:PermSize=10m -XX:MaxPermSize=10m
*
* jdk8中：
* -XX:MetaspaceSize=10m -XX:MaxMetaspaceSize=10m
*
* @author shkstart [email protected]
* @create 2020 22:24
*/
public class OOMTest extends ClassLoader {
public static void main(String[] args) {
int j = 0;
try {
OOMTest test = new OOMTest();
for (int i = 0; i < 10000; i++) {
//创建ClassWriter对象，用于生成类的二进制字节码
ClassWriter classWriter = new ClassWriter(0);
//指明版本号，修饰符，类名，包名，父类，接口
classWriter.visit(Opcodes.V1_6, Opcodes.ACC_PUBLIC, "Class" + i, null, "java/lang/Object", null);
//返回byte[]
byte[] code = classWriter.toByteArray();
//类的加载
test.defineClass("Class" + i, code, 0, code.length);//Class对象
j++;
}
} finally {
System.out.println(j);
}
}
}
3、如何解决 OOM
4、方法区的内部结构
字节码中对应的方法区信息
package com.atguigu.java;
import java.io.Serializable;
/**
* 测试方法区的内部构成
* @author shkstart [email protected]
* @create 2020 23:39
*/
public class MethodInnerStrucTest extends Object implements Comparable<String>,Serializable {
//属性
public int num = 10;
private static String str = "测试方法的内部结构";
//构造器
//方法
public void test1(){
int count = 20;
System.out.println("count = " + count);
}
public static int test2(int cal){
int result = 0;
try {
int value = 30;
result = value / cal;
} catch (Exception e) {
e.printStackTrace();
}
return result;
}
@Override
public int compareTo(String o) {
return 0;
}
}
Classfile /D:/workspace_idea5/JVMDemo/out/production/chapter09/com/atguigu/java/MethodInnerStrucTest.class
Last modified 2020-4-22; size 1626 bytes
MD5 checksum 69643a16925bb67a96f54050375c75d0
Compiled from "MethodInnerStrucTest.java"
//类型信息
public class com.atguigu.java.MethodInnerStrucTest extends java.lang.Object
implements java.lang.Comparable<java.lang.String>, java.io.Serializable
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #18.#52 // java/lang/Object."":()V
#2 = Fieldref #17.#53 // com/atguigu/java/MethodInnerStrucTest.num:I
#3 = Fieldref #54.#55 // java/lang/System.out:Ljava/io/PrintStream;
#4 = Class #56 // java/lang/StringBuilder
#5 = Methodref #4.#52 // java/lang/StringBuilder."":()V
#6 = String #57 // count =
#7 = Methodref #4.#58 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#8 = Methodref #4.#59 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#9 = Methodref #4.#60 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#10 = Methodref #61.#62 // java/io/PrintStream.println:(Ljava/lang/String;)V
#11 = Class #63 // java/lang/Exception
#12 = Methodref #11.#64 // java/lang/Exception.printStackTrace:()V
#13 = Class #65 // java/lang/String
#14 = Methodref #17.#66 // com/atguigu/java/MethodInnerStrucTest.compareTo:(Ljava/lang/String;)I
#15 = String #67 // 测试方法的内部结构
#16 = Fieldref #17.#68 // com/atguigu/java/MethodInnerStrucTest.str:Ljava/lang/String;
#17 = Class #69 // com/atguigu/java/MethodInnerStrucTest
#18 = Class #70 // java/lang/Object
#19 = Class #71 // java/lang/Comparable
#20 = Class #72 // java/io/Serializable
#21 = Utf8 num
#22 = Utf8 I
#23 = Utf8 str
#24 = Utf8 Ljava/lang/String;
#25 = Utf8 <init>
#26 = Utf8 ()V
#27 = Utf8 Code
#28 = Utf8 LineNumberTable
#29 = Utf8 LocalVariableTable
#30 = Utf8 this
#31 = Utf8 Lcom/atguigu/java/MethodInnerStrucTest;
#32 = Utf8 test1
#33 = Utf8 count
#34 = Utf8 test2
#35 = Utf8 (I)I
#36 = Utf8 value
#37 = Utf8 e
#38 = Utf8 Ljava/lang/Exception;
#39 = Utf8 cal
#40 = Utf8 result
#41 = Utf8 StackMapTable
#42 = Class #63 // java/lang/Exception
#43 = Utf8 compareTo
#44 = Utf8 (Ljava/lang/String;)I
#45 = Utf8 o
#46 = Utf8 (Ljava/lang/Object;)I
#47 = Utf8 <clinit>
#48 = Utf8 Signature
#49 = Utf8 Ljava/lang/Object;Ljava/lang/Comparable<Ljava/lang/String;>;Ljava/io/Serializable;
#50 = Utf8 SourceFile
#51 = Utf8 MethodInnerStrucTest.java
#52 = NameAndType #25:#26 // "":()V
#53 = NameAndType #21:#22 // num:I
#54 = Class #73 // java/lang/System
#55 = NameAndType #74:#75 // out:Ljava/io/PrintStream;
#56 = Utf8 java/lang/StringBuilder
#57 = Utf8 count =
#58 = NameAndType #76:#77 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#59 = NameAndType #76:#78 // append:(I)Ljava/lang/StringBuilder;
#60 = NameAndType #79:#80 // toString:()Ljava/lang/String;
#61 = Class #81 // java/io/PrintStream
#62 = NameAndType #82:#83 // println:(Ljava/lang/String;)V
#63 = Utf8 java/lang/Exception
#64 = NameAndType #84:#26 // printStackTrace:()V
#65 = Utf8 java/lang/String
#66 = NameAndType #43:#44 // compareTo:(Ljava/lang/String;)I
#67 = Utf8 测试方法的内部结构
#68 = NameAndType #23:#24 // str:Ljava/lang/String;
#69 = Utf8 com/atguigu/java/MethodInnerStrucTest
#70 = Utf8 java/lang/Object
#71 = Utf8 java/lang/Comparable
#72 = Utf8 java/io/Serializable
#73 = Utf8 java/lang/System
#74 = Utf8 out
#75 = Utf8 Ljava/io/PrintStream;
#76 = Utf8 append
#77 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#78 = Utf8 (I)Ljava/lang/StringBuilder;
#79 = Utf8 toString
#80 = Utf8 ()Ljava/lang/String;
#81 = Utf8 java/io/PrintStream
#82 = Utf8 println
#83 = Utf8 (Ljava/lang/String;)V
#84 = Utf8 printStackTrace
{
//域信息
public int num;
descriptor: I
flags: ACC_PUBLIC
private static java.lang.String str;
descriptor: Ljava/lang/String;
flags: ACC_PRIVATE, ACC_STATIC
//方法信息
public com.atguigu.java.MethodInnerStrucTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: aload_0
5: bipush 10
7: putfield #2 // Field num:I
10: return
LineNumberTable:
line 10: 0
line 12: 4
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lcom/atguigu/java/MethodInnerStrucTest;
public void test1();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=3, locals=2, args_size=1
0: bipush 20
2: istore_1
3: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
6: new #4 // class java/lang/StringBuilder
9: dup
10: invokespecial #5 // Method java/lang/StringBuilder."":()V
13: ldc #6 // String count =
15: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
18: iload_1
19: invokevirtual #8 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
22: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
25: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
28: return
LineNumberTable:
line 17: 0
line 18: 3
line 19: 28
LocalVariableTable:
Start Length Slot Name Signature
0 29 0 this Lcom/atguigu/java/MethodInnerStrucTest;
3 26 1 count I
public static int test2(int);
descriptor: (I)I
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=1
0: iconst_0
1: istore_1
2: bipush 30
4: istore_2
5: iload_2
6: iload_0
7: idiv
8: istore_1
9: goto 17
12: astore_2
13: aload_2
14: invokevirtual #12 // Method java/lang/Exception.printStackTrace:()V
17: iload_1
18: ireturn
Exception table:
from to target type
2 9 12 Class java/lang/Exception
LineNumberTable:
line 21: 0
line 23: 2
line 24: 5
line 27: 9
line 25: 12
line 26: 13
line 28: 17
LocalVariableTable:
Start Length Slot Name Signature
5 4 2 value I
13 4 2 e Ljava/lang/Exception;
0 19 0 cal I
2 17 1 result I
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 12
locals = [ int, int ]
stack = [ class java/lang/Exception ]
frame_type = 4 /* same */
public int compareTo(java.lang.String);
descriptor: (Ljava/lang/String;)I
flags: ACC_PUBLIC
Code:
stack=1, locals=2, args_size=2
0: iconst_0
1: ireturn
LineNumberTable:
line 33: 0
LocalVariableTable:
Start Length Slot Name Signature
0 2 0 this Lcom/atguigu/java/MethodInnerStrucTest;
0 2 1 o Ljava/lang/String;
public int compareTo(java.lang.Object);
descriptor: (Ljava/lang/Object;)I
flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: aload_1
2: checkcast #13 // class java/lang/String
5: invokevirtual #14 // Method compareTo:(Ljava/lang/String;)I
8: ireturn
LineNumberTable:
line 10: 0
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lcom/atguigu/java/MethodInnerStrucTest;
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: ldc #15 // String 测试方法的内部结构
2: putstatic #16 // Field str:Ljava/lang/String;
5: return
LineNumberTable:
line 13: 0
}
Signature: #49 // Ljava/lang/Object;Ljava/lang/Comparable;Ljava/io/Serializable;
SourceFile: "MethodInnerStrucTest.java"
5、non-final 的类变量
package com.atguigu.java;
/**
* non-final的类变量
* @author shkstart [email protected]
* @create 2020 20:37
*/
public class MethodAreaTest {
public static void main(String[] args) {
Order order = null;
//为 null 也能调用静态方法
order.hello();
System.out.println(order.count);
}
}
class Order {
public static int count = 1;
public static final int number = 2;
public static void hello() {
System.out.println("hello!");
}
}
四、 运行时常量池
1、常量池
2、运行时常量池
3、方法区使用举例
package com.atguigu.java1;
/**
* @author shkstart [email protected]
* @create 2020 14:28
*/
public class MethodAreaDemo {
public static void main(String[] args) {
int x = 500;
int y = 100;
int a = x / y;
int b = 50;
System.out.println(a + b);
}
}
4、方法区的演进细节
5、永久代为什么要被元空间替换
package com.atguigu.java1;
/**
* 结论： 只要是 new 的对象，都在堆空间
* 静态引用对应的对象实体始终都存在堆空间
*
* jdk7：
* -Xms200m -Xmx200m -XX:PermSize=300m -XX:MaxPermSize=300m -XX:+PrintGCDetails
* jdk 8：
* -Xms200m -Xmx200m -XX:MetaspaceSize=300m -XX:MaxMetaspaceSize=300m -XX:+PrintGCDetails
* @author shkstart [email protected]
* @create 2020 21:20
*/
public class StaticFieldTest {
private static byte[] arr = new byte[1024 * 1024 * 100];//100MB
public static void main(String[] args) {
System.out.println(StaticFieldTest.arr);
// try {
// Thread.sleep(1000000);
// } catch (InterruptedException e) {
// e.printStackTrace();
// }
}
}
6、测试对象分别存在哪些地方
package com.atguigu.java1;
/**
* 《深入理解Java虚拟机》中的案例：
* staticObj、instanceObj、localObj存放在哪里？ 只要是 new 出来的对象实例都在堆区
* @author shkstart [email protected]
* @create 2020 11:39
*/
public class StaticObjTest {
static class Test {
//静态变量在方法区
static ObjectHolder staticObj = new ObjectHolder();
//成员变量在堆区
ObjectHolder instanceObj = new ObjectHolder();
void foo() {
//方法内的局部变量在 局部变量表
ObjectHolder localObj = new ObjectHolder();
System.out.println("done");
}
}
private static class ObjectHolder {
}
public static void main(String[] args) {
Test test = new StaticObjTest.Test();
test.foo();
}
}
7、方法区的垃圾回收
五、 总结
