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Exporting Your Beans to JMX

The core class in Spring’s JMX framework is the MBeanExporter. This class is responsible for taking your Spring beans and registering them with a JMX MBeanServer. For example, consider the following class:

package org.springframework.jmx;

public class JmxTestBean implements IJmxTestBean {

	private String name;
	private int age;
	private boolean isSuperman;

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

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

	public void dontExposeMe() {
		throw new RuntimeException();
	}
}

To expose the properties and methods of this bean as attributes and operations of an MBean, you can configure an instance of the MBeanExporter class in your configuration file and pass in the bean, as the following example shows:

<beans>
	<!-- this bean must not be lazily initialized if the exporting is to happen -->
	<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter" lazy-init="false">
		<property name="beans">
			<map>
				<entry key="bean:name=testBean1" value-ref="testBean"/>
			</map>
		</property>
	</bean>
	<bean id="testBean" class="org.springframework.jmx.JmxTestBean">
		<property name="name" value="TEST"/>
		<property name="age" value="100"/>
	</bean>
</beans>

The pertinent bean definition from the preceding configuration snippet is the exporter bean. The beans property tells the MBeanExporter exactly which of your beans must be exported to the JMX MBeanServer. In the default configuration, the key of each entry in the beans Map is used as the ObjectName for the bean referenced by the corresponding entry value. You can change this behavior, as described in Controlling ObjectName Instances for Your Beans.

With this configuration, the testBean bean is exposed as an MBean under the ObjectName bean:name=testBean1. By default, all public properties of the bean are exposed as attributes and all public methods (except those inherited from the Object class) are exposed as operations.

MBeanExporter is a Lifecycle bean (see Startup and Shutdown Callbacks ). By default, MBeans are exported as late as possible during the application lifecycle. You can configure the phase at which the export happens or disable automatic registration by setting the autoStartup flag.

Creating an MBeanServer

The configuration shown in the preceding section assumes that the application is running in an environment that has one (and only one) MBeanServer already running. In this case, Spring tries to locate the running MBeanServer and register your beans with that server (if any). This behavior is useful when your application runs inside a container (such as Tomcat or IBM WebSphere) that has its own MBeanServer.

However, this approach is of no use in a standalone environment or when running inside a container that does not provide an MBeanServer. To address this, you can create an MBeanServer instance declaratively by adding an instance of the org.springframework.jmx.support.MBeanServerFactoryBean class to your configuration. You can also ensure that a specific MBeanServer is used by setting the value of the MBeanExporter instance’s server property to the MBeanServer value returned by an MBeanServerFactoryBean, as the following example shows:

<beans>

	<bean id="mbeanServer" class="org.springframework.jmx.support.MBeanServerFactoryBean"/>

	<!--
	this bean needs to be eagerly pre-instantiated in order for the exporting to occur;
	this means that it must not be marked as lazily initialized
	-->
	<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
		<property name="beans">
			<map>
				<entry key="bean:name=testBean1" value-ref="testBean"/>
			</map>
		</property>
		<property name="server" ref="mbeanServer"/>
	</bean>

	<bean id="testBean" class="org.springframework.jmx.JmxTestBean">
		<property name="name" value="TEST"/>
		<property name="age" value="100"/>
	</bean>

</beans>

In the preceding example, an instance of MBeanServer is created by the MBeanServerFactoryBean and is supplied to the MBeanExporter through the server property. When you supply your own MBeanServer instance, the MBeanExporter does not try to locate a running MBeanServer and uses the supplied MBeanServer instance. For this to work correctly, you must have a JMX implementation on your classpath.

Reusing an Existing `MBeanServer`

If no server is specified, the MBeanExporter tries to automatically detect a running MBeanServer. This works in most environments, where only one MBeanServer instance is used. However, when multiple instances exist, the exporter might pick the wrong server. In such cases, you should use the MBeanServer agentId to indicate which instance to be used, as the following example shows:

<beans>
	<bean id="mbeanServer" class="org.springframework.jmx.support.MBeanServerFactoryBean">
		<!-- indicate to first look for a server -->
		<property name="locateExistingServerIfPossible" value="true"/>
		<!-- search for the MBeanServer instance with the given agentId -->
		<property name="agentId" value="MBeanServer_instance_agentId>"/>
	</bean>
	<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
		<property name="server" ref="mbeanServer"/>
		...
	</bean>
</beans>

For platforms or cases where the existing MBeanServer has a dynamic (or unknown) agentId that is retrieved through lookup methods, you should use factory-method, as the following example shows:

<beans>
	<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
		<property name="server">
			<!-- Custom MBeanServerLocator -->
			<bean class="platform.package.MBeanServerLocator" factory-method="locateMBeanServer"/>
		</property>
	</bean>

	<!-- other beans here -->

</beans>

Lazily Initialized MBeans

If you configure a bean with an MBeanExporter that is also configured for lazy initialization, the MBeanExporter does not break this contract and avoids instantiating the bean. Instead, it registers a proxy with the MBeanServer and defers obtaining the bean from the container until the first invocation on the proxy occurs.

This also affects FactoryBean resolution where MBeanExporter will regularly introspect the produced object, effectively triggering FactoryBean.getObject(). In order to avoid this, mark the corresponding bean definition as lazy-init.

Automatic Registration of MBeans

Any beans that are exported through the MBeanExporter and are already valid MBeans are registered as-is with the MBeanServer without further intervention from Spring. You can cause MBeans to be automatically detected by the MBeanExporter by setting the autodetect property to true, as the following example shows:

<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
	<property name="autodetect" value="true"/>
</bean>

<bean name="spring:mbean=true" class="org.springframework.jmx.export.TestDynamicMBean"/>

In the preceding example, the bean called spring:mbean=true is already a valid JMX MBean and is automatically registered by Spring. By default, a bean that is autodetected for JMX registration has its bean name used as the ObjectName. You can override this behavior, as detailed in Controlling ObjectName Instances for Your Beans.

Controlling the Registration Behavior

Consider the scenario where a Spring MBeanExporter attempts to register an MBean with an MBeanServer by using the ObjectName bean:name=testBean1. If an MBean instance has already been registered under that same ObjectName, the default behavior is to fail (and throw an InstanceAlreadyExistsException).

You can control exactly what happens when an MBean is registered with an MBeanServer. Spring’s JMX support allows for three different registration behaviors to control the registration behavior when the registration process finds that an MBean has already been registered under the same ObjectName. The following table summarizes these registration behaviors:

Table 1. Registration Behaviors
Registration behavior	Explanation
`FAIL_ON_EXISTING`	This is the default registration behavior. If an `MBean` instance has already been registered under the same `ObjectName`, the `MBean` that is being registered is not registered, and an `InstanceAlreadyExistsException` is thrown. The existing `MBean` is unaffected.
`IGNORE_EXISTING`	If an `MBean` instance has already been registered under the same `ObjectName`, the `MBean` that is being registered is not registered. The existing `MBean` is unaffected, and no `Exception` is thrown. This is useful in settings where multiple applications want to share a common `MBean` in a shared `MBeanServer`.
`REPLACE_EXISTING`	If an `MBean` instance has already been registered under the same `ObjectName`, the existing `MBean` that was previously registered is unregistered, and the new `MBean` is registered in its place (the new `MBean` effectively replaces the previous instance).

The values in the preceding table are defined as enums on the RegistrationPolicy class. If you want to change the default registration behavior, you need to set the value of the registrationPolicy property on your MBeanExporter definition to one of those values.

The following example shows how to change from the default registration behavior to the REPLACE_EXISTING behavior:

<beans>

	<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter">
		<property name="beans">
			<map>
				<entry key="bean:name=testBean1" value-ref="testBean"/>
			</map>
		</property>
		<property name="registrationPolicy" value="REPLACE_EXISTING"/>
	</bean>

	<bean id="testBean" class="org.springframework.jmx.JmxTestBean">
		<property name="name" value="TEST"/>
		<property name="age" value="100"/>
	</bean>

</beans>