How to Generate JVM Heap Memory Dump?

Java Heap dump is a snapshot of all java objects that are present in the JVM(Java Virtual Machine) at a certain point in time. The JVM allocates memory for objects which are class instances or arrays in the heap memory. When the objects are no longer needed or are no more referenced, the Garbage Collector runs and reclaims the memory space occupied by these objects.

The heap dump is in binary format, and it has .hprof extension. It can be opened and analyzed using applications like JVisualVM and Eclipse MAT(Memory Analyzer Tool). We generate java memory heap dump to identify issues like memory leaks and to optimize memory usage in our application.

Methods:

There are different ways of generating a java memory heap dump. JDK comes up with various tools for generating heap dump. These tools are located in bin folder under JDK home directory.. Let us discuss how to generate JVM Heap Dump which is as follows:

1. Using jmap command
2. Using jcmd command on terminal
3. Using the JVisualVM tool
4. Identifying HeapDumpOnOutOfMemory
5. Using JMX Console
6. Using HotSpotDiagnosticMBean by writing a program

Method 1: Using map command

jmap is a command which you run inside the bin folder of your JDK home directory. It gives statistics about memory usage. The structure is as follows:

Example

jmap -dump:[live],format=b,file=<file-path> <pid>

live:- This parameter is optional. If set, it prints all those objects that 
have active references.

format = b , which means the heap dump file is in binary format. It is not necessary 
to set this parameter.

file =<file-path> indicates where the heap dump file will be generated.

<pid> :- process id of the java process

Now in order to get the process id of a running java process, one can use one of the below options as defined:

1.1

jps

We type this command from a Unix terminal or Windows Command prompt where JDK is installed. It gives the process ID of the running java process

jps command

1.2

ps -eaf| grep java

This gives the process ID of all running java processes. It works only on a Unix Terminal

ps -eaf | grep java

1.3 Using task manager application in windows operating systems.

Method 2: Using jcmd command on terminal

This command sends a request to the JVM to generate a heap dump. One of its parameters is GC.heap_dump. It is as shown below:

jcmd <pid> GC.heap_dump <file-path>
<pid> - Process id of java process
<file-path> - Path where the heap dump is to be generated

jcmd

Method 3: Using the JVisualVM tool 

This is a tool that is packaged within JDK. It helps to monitor and troubleshoot java applications.  It has a  Graphical User Interface that is simple and intuitive. We type jvisualvm in the start menu, or we go to the bin directory of JDK home directory through command prompt or terminal window in Unix and type jvisualvm

It launches a Java Visual VM application. On the left side, it shows the currently running java process. Right-click the process ID whose heap dump you wish to generate. When we click on heap dump, it generates heap dump for the selected process. Under Basic info, it shows the file path where the heap dump is generated.

Method 4: Identifying HeapDumpOnOutOfMemory

It is ideal to capture heap dumps when an application experiences java.lang.OutOfMemoryError. Heap dumps help identify live objects sitting in the memory and the percentage of memory it occupies. 

-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=<file-path>

While running your java application when this system property is set, JVM takes a snapshot of the heap dump when it encounters OutOfMemoryError. 

Method 5: Using JMX Console

There is a Hotspot Diagnostic MBean which has a heap dump operation. We use jmx clients like jconsole to invoke the MBean operation. Through JConsole, we can either connect to a local java process or a remote process by specifying the host and port number and username/password. Once we connect to the process id, jconsole applications open with multiple tabs. The Overview tab shows Heap Memory usage, threads, classes, CPU usage

Jconsole – new connection

Jconsole – MBeans tab

Method 6: Using HotSpotDiagnosticMBean by writing a program 

We use HotSpotDiagnosticMBean to invoke the heap dump operation. We get the MBean object from the MBean platform server.  In the below example, we have used reflection to invoke the heapDump() method of the MBean object.

Example

Java

// Java Program Invoking heapDump() Method of MBean Object

// Importing input output classes
import java.io.*;

// Main class
class GFG {

    // Custom string passed as input
    private static final String HOTSPOT_BEAN
        = "com.sun.management:type=HotSpotDiagnostic";

    // Private member variable of this class
    private static volatile Object hotSpotMBean;

    // Invoke this method when heap dump is to be generated
    // @param fileName - name of the heap dump file
    // @param live - indicates if only live objects are to
    // be included in the heap dump

    // Method 1
    // To generate heap dumps
    static void generateHeapDump(String fileName,
                                 boolean live)
    {
        initHotspotMBean();

        // Try block to check if any exceptions occurs
        try {

            Class clazz = Class.forName(
                "com.sun.management.HotSpotDiagnosticMXBean");
            Method m = clazz.getMethod(
                "dumpHeap", String.class, boolean.class);
            m.invoke(hotSpotMBean, fileName, live);
        }

        // Catch block handling runtime exceptions
        catch (RuntimeException re) {
            throw re;
        }
        catch (Exception exp) {
            throw new RuntimeException(exp);
        }
    }

    // Method 2
    private static void initHotspotMBean()
    {

        if (hotSpotMBean == null) {
            synchronized (JavaHeapDump.class)
            {

                if (hotSpotMBean == null) {
                    hotSpotMBean = getHotSpotMbean();
                }
            }
        }
    }

    // Method 3
    // To get the HOtSpotBean from the MBean server
    private static Object getHotSpotMbean()
    {

        Object hotspotBean = null;

        // Try block tocheck for exceptions
        try {

            // Loading using .forName() method
            Class clazz = Class.forName(
                "com.sun.management.HotSpotDiagnosticMXBean");

            MBeanServer mbeanServer
                = ManagementFactory
                      .getPlatformMBeanServer();

            hotspotBean
                = ManagementFactory.newPlatformMXBeanProxy(
                    mbeanServer, HOTSPOT_BEAN, clazz);

            return hotspotBean;
        }

        // Catch block 1
        // Handling exceptions if class not found
        catch (ClassNotFoundException e) {

            // Printthe exception along with line number
            // using printStackTrace() method
            e.printStackTrace();
        }

        // Catch block 2
        // Handling basic I/O exceptions
        catch (IOException e) {

            // Printthe exception along with line number
            // using printStackTrace() method
            e.printStackTrace();
        }

        return hotspotBean;
    }

    // Method 4
    // Main driver method
    public static void main(String[] args)
    {

        // File taken as an input
        String fileName
            = "/home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/JavaHeapDumpGenerator/src/heap1.hprof";

        // Flag variable set to true
        boolean live = true;

        // Switch case
        switch (args.length) {
        case 2:
            live = args[1].equals("true");
        case 1:
            fileName = args[0];
        }

        // Calling Method 1 in main() method to
        // generate heap dumps
        generateHeapDump(fileName, live);
    }
}

Note:

We run this application by passing command-line arguments for file path where the heap dump is to be generated and live parameter which can be set as true or false. When this java code is run, it generates a heap1.hprof file in src folder. This heap dump can be analyzed using MAT(Memory Analyzer Tool). This can be installed as a plugin in Eclipse from Marketplace.

Heap Dump Analysis using MAT

• Now lastly let us analyze the heap dump file with help of JVisualVM.
• Once a heap dump file is generated, we use tools like JVisualVM to analyze the file. When you open a heap dump, Java VisualVM displays the Summary view by default. The Summary view displays the running environment where the heap dump was taken and other system properties.
• In JvisualVM, we go to File -> Load and select the folder location where the ‘.hprof file’ is generated which is pictorially aided below to get a fair understanding for the same.

Analysis of heap dump file – Summary tab

Analysis of heap dump – Classes tab