JMX Attribute Access

import com.codahale.metrics.jvm.JmxAttributeGauge;
import com.codahale.metrics.MetricRegistry;
import javax.management.ObjectName;
import javax.management.MBeanServerConnection;

/**
 * A Gauge implementation which queries an MBeanServerConnection for an attribute of an object.
 */
public class JmxAttributeGauge implements Gauge<Object> {
    /**
     * Creates a new JmxAttributeGauge using the platform MBean server.
     * @param objectName the name of the object
     * @param attributeName the name of the object's attribute
     */
    public JmxAttributeGauge(ObjectName objectName, String attributeName);
    
    /**
     * Creates a new JmxAttributeGauge with a custom MBeanServerConnection.
     * @param mBeanServerConn the MBeanServerConnection
     * @param objectName the name of the object
     * @param attributeName the name of the object's attribute
     */
    public JmxAttributeGauge(MBeanServerConnection mBeanServerConn, ObjectName objectName, String attributeName);
    
    /**
     * Gets the current value of the JMX attribute.
     * @return the attribute value, or null if unavailable
     */
    public Object getValue();
}

import com.codahale.metrics.MetricRegistry;
import com.codahale.metrics.jvm.JmxAttributeGauge;
import javax.management.ObjectName;
import java.lang.management.ManagementFactory;

// Basic usage with platform MBean server
MetricRegistry registry = new MetricRegistry();

// Monitor JVM memory usage
ObjectName memoryName = new ObjectName("java.lang:type=Memory");
JmxAttributeGauge heapUsed = new JmxAttributeGauge(memoryName, "HeapMemoryUsage");
registry.register("jvm.memory.heap.composite", heapUsed);

// Monitor specific memory pool
ObjectName poolName = new ObjectName("java.lang:type=MemoryPool,name=PS Eden Space");
JmxAttributeGauge poolUsage = new JmxAttributeGauge(poolName, "Usage");
registry.register("jvm.memory.pool.eden.usage", poolUsage);

// Monitor garbage collector information
ObjectName gcName = new ObjectName("java.lang:type=GarbageCollector,name=PS Scavenge");
JmxAttributeGauge gcCount = new JmxAttributeGauge(gcName, "CollectionCount");
JmxAttributeGauge gcTime = new JmxAttributeGauge(gcName, "CollectionTime");
registry.register("jvm.gc.ps-scavenge.count", gcCount);
registry.register("jvm.gc.ps-scavenge.time", gcTime);

// Access attribute values
Object heapUsage = heapUsed.getValue();
System.out.println("Heap usage: " + heapUsage);

Long collectionCount = (Long) gcCount.getValue();
Long collectionTime = (Long) gcTime.getValue();
System.out.println("GC collections: " + collectionCount);
System.out.println("GC time: " + collectionTime + " ms");

// Custom MBeanServerConnection
MBeanServerConnection customConnection = ManagementFactory.getPlatformMBeanServer();
ObjectName customName = new ObjectName("com.example:type=CustomMetric");
JmxAttributeGauge customGauge = new JmxAttributeGauge(customConnection, customName, "Value");
registry.register("custom.metric", customGauge);

// Pattern-based ObjectName for dynamic discovery
ObjectName pattern = new ObjectName("java.lang:type=MemoryPool,name=*");
JmxAttributeGauge dynamicGauge = new JmxAttributeGauge(pattern, "Usage");

// When multiple MBeans match the pattern, the first one found is used
// For single-match scenarios, this provides flexibility in MBean naming

// Memory and GC monitoring
new ObjectName("java.lang:type=Memory");
new ObjectName("java.lang:type=MemoryPool,name=*");
new ObjectName("java.lang:type=GarbageCollector,name=*");

// Thread monitoring
new ObjectName("java.lang:type=Threading");

// Class loading
new ObjectName("java.lang:type=ClassLoading");

// Runtime information
new ObjectName("java.lang:type=Runtime");

// Operating system
new ObjectName("java.lang:type=OperatingSystem");

// Buffer pools (Java 7+)
new ObjectName("java.nio:type=BufferPool,name=*");

// Application-specific MBeans
new ObjectName("com.myapp:type=DatabasePool,name=primary");
new ObjectName("com.myapp:type=Cache,name=*");

// Robust error handling example
public class SafeJmxAttributeGauge extends JmxAttributeGauge {
    private final String description;
    
    public SafeJmxAttributeGauge(ObjectName objectName, String attributeName, String description) {
        super(objectName, attributeName);
        this.description = description;
    }
    
    @Override
    public Object getValue() {
        try {
            Object value = super.getValue();
            if (value == null) {
                System.err.println("Warning: " + description + " returned null");
            }
            return value;
        } catch (Exception e) {
            System.err.println("Error reading " + description + ": " + e.getMessage());
            return null;
        }
    }
}

// Usage with error handling
SafeJmxAttributeGauge safeGauge = new SafeJmxAttributeGauge(
    new ObjectName("java.lang:type=Memory"),
    "HeapMemoryUsage",
    "JVM Heap Memory Usage"
);

import javax.management.openmbean.CompositeData;

JmxAttributeGauge memoryUsage = new JmxAttributeGauge(
    new ObjectName("java.lang:type=Memory"), 
    "HeapMemoryUsage"
);

Object value = memoryUsage.getValue();
if (value instanceof CompositeData) {
    CompositeData composite = (CompositeData) value;
    Long used = (Long) composite.get("used");
    Long max = (Long) composite.get("max");
    Long committed = (Long) composite.get("committed");
    Long init = (Long) composite.get("init");
    
    System.out.println("Heap used: " + used);
    System.out.println("Heap max: " + max);
    System.out.println("Heap committed: " + committed);
    System.out.println("Heap init: " + init);
}