MBean，managed bean，被管理的bean，也就是一个被管理的Java对象。
它暴露了一个管理接口，可以包含以下内容：

• 一系列可读或可写的属性
• 一系列可调用的操作
• 自我描述

SpringApplicationAdminMXBean

Springboot里默认有暴露出SpringApplicationAdminMXBean，通过源码可以看出，是在afterPropertiesSet方法注册了mbean，destroy方法取消注册

@Override
public void afterPropertiesSet() throws Exception {
    MBeanServer server = ManagementFactory.getPlatformMBeanServer();
    server.registerMBean(new SpringApplicationAdmin(), this.objectName);
    if (logger.isDebugEnabled()) {
        logger.debug("Application Admin MBean registered with name '" + this.objectName + "'");
    }
}

@Override
public void destroy() throws Exception {
    ManagementFactory.getPlatformMBeanServer().unregisterMBean(this.objectName);
}

该bean本身暴露了两个属性(Ready, EmbeddedWebApplication)，两个方法(getProperty, shutdown)

private class SpringApplicationAdmin implements SpringApplicationAdminMXBean {

        @Override
        public boolean isReady() {
            return SpringApplicationAdminMXBeanRegistrar.this.ready;
        }

        @Override
        public boolean isEmbeddedWebApplication() {
            return SpringApplicationAdminMXBeanRegistrar.this.embeddedWebApplication;
        }

        @Override
        public String getProperty(String key) {
            return SpringApplicationAdminMXBeanRegistrar.this.environment.getProperty(key);
        }

        @Override
        public void shutdown() {
            logger.info("Application shutdown requested.");
            SpringApplicationAdminMXBeanRegistrar.this.applicationContext.close();
        }

    }

通过jvisualvm，可以看到具体mbean的信息

jvisualvm-Admin

自己实现

我们自己也可以仿照Admin类，自己实现一个MBean
代码如下，定义了一个MBean: cn.ye:type=Hello

public interface HelloMBean {

    public void sayHello();

    public int add(int x, int y);

    public String getName();

    public int getCacheSize();

    public void setCacheSize(int size);
}

public class Hello implements HelloMBean {

    private final String name = "Reginald";
    private int cacheSize = DEFAULT_CACHE_SIZE;
    private static final int DEFAULT_CACHE_SIZE = 200;

    @Override
    public void sayHello() {
        System.out.println("hello, world");
    }

    @Override
    public int add(int x, int y) {
        return x + y;
    }

    @Override
    public String getName() {
        return this.name;
    }

    @Override
    public int getCacheSize() {
        return this.cacheSize;
    }

    @Override
    public synchronized void setCacheSize(int size) {
        this.cacheSize = size;
        System.out.println("Cache size now " + this.cacheSize);
    }
}

启动类里，注册MBean

public class Application {

    public static void main(String[] args) throws MalformedObjectNameException, NotCompliantMBeanException, InstanceAlreadyExistsException, MBeanRegistrationException {
        SpringApplication.run(Application.class, args);
        MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
        ObjectName name = new ObjectName("cn.ye:type=Hello");
        Hello mbean = new Hello();
        mbs.registerMBean(mbean, name);
    }
}

启动jvisualvm，可以发现mbean已经注册上去了

其中CacheSize可修改，Name只读

Hello

操作里，也有两个方法，可以正常调用

Hello-Operations