JVM专题之内存模型以及如何判定对象已死问题

体验与验证

2.4.5.1 使用visualvm

**visualgc插件下载链接 ：https://visualvm.github.io/pluginscenters.html

https://visualvm.github.io/pluginscenters.html

**选择对应JDK版本链接--->Tools--->Visual GC**
 

2.4.5.2 堆内存溢出

* **代码**

java
@RestController
public class HeapController {
    List<Person> list=new ArrayList<Person>();
    @GetMapping("/heap")
    public String heap(){
        while(true){
            list.add(new Person());
        }
    }
}
```

记得设置参数比如-Xmx20M -Xms20M**

* **运行结果**

`访问`：http://localhost:8080/heap  http://localhost:8080/heap

```
Exception in thread "http-nio-8080-exec-2" java.lang.OutOfMemoryError: GC overhead limit exceeded
```

2.4.5.3 方法区内存溢出

> **比如向方法区中添加Class的信息**

* **asm依赖和Class代码**

```xml
<dependency>
    <groupId>asm</groupId>
    <artifactId>asm</artifactId>
    <version>3.3.1</version>
</dependency>
```

```java
public class MyMetaspace extends ClassLoader {
    public static List<Class<?>> createClasses() {
        List<Class<?>> classes = new ArrayList<Class<?>>();
        for (int i = 0; i < 10000000; ++i) {
            ClassWriter cw = new ClassWriter(0);
            cw.visit(Opcodes.V1_1, Opcodes.ACC_PUBLIC, "Class" + i, null,
                    "java/lang/Object", null);
            MethodVisitor mw = cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>",
                    "()V", null, null);
            mw.visitVarInsn(Opcodes.ALOAD, 0);
            mw.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object",
                    "<init>", "()V");
            mw.visitInsn(Opcodes.RETURN);
            mw.visitMaxs(1, 1);
            mw.visitEnd();
            Metaspace test = new Metaspace();
            byte[] code = cw.toByteArray();
            Class<?> exampleClass = test.defineClass("Class" + i, code, 0, code.length);
            classes.add(exampleClass);
        }
        return classes;
    }
}
```

* **代码**

```java
@RestController
public class