I have a Java application that I run from a console which in turn executes an another Java process. I want to get a thread/heap dump of that child process.
On Unix, I could do a kill -3 <pid> but on Windows AFAIK the only way to get a thread dump is Ctrl-Break in the console. But that only gives me the dump of the parent process, not the child.
Is there another way to get that heap dump?
You can use jmap to get a dump of any process running, assuming you know the pid.
Use Task Manager or Resource Monitor to get the pid. Then
jmap -dump:format=b,file=heap.hprof <pid>
to get the heap for that process.
For systems where bash and pgrep are installed and a single Java process is running, try:
jmap -dump:format=b,file=heap.hprof $(pgrep java)
You are confusing two different java dumps. kill -3 generates a thread dump, not a heap dump.
Thread dump = stack traces for each thread in the JVM output to stdout as text.
Heap dump = memory contents for the JVM process output to a binary file.
To take a thread dump on Windows, CTRL+BREAK if your JVM is the foreground process is the simplest way. If you have a unix-like shell on Windows like Cygwin or MobaXterm, you can use kill -3 {pid} like you can in Unix.
To take a thread dump in Unix, CTRL+C if your JVM is the foreground process or kill -3 {pid} will work as long as you get the right PID for the JVM.
With either platform, Java comes with several utilities that can help. For thread dumps, jstack {pid} is your best bet. http://docs.oracle.com/javase/1.5.0/docs/tooldocs/share/jstack.html
Just to finish the dump question out: Heap dumps are not commonly used because they are difficult to interpret. But, they have a lot of useful information in them if you know where/how to look at them. The most common usage is to locate memory leaks. It is a good practice to set the -D on the java command-line so that the heap dump is generated automatically upon an OutOfMemoryError, -XX:+HeapDumpOnOutOfMemoryError But, you can manually trigger a heap dump, also. The most common way is to use the java utility jmap.
NOTE: this utility is not available on all platforms. As of JDK 1.6, jmap is available on Windows.
An example command-line would look something like
jmap -dump:file=myheap.bin {pid of the JVM}
The output "myheap.bin" is not human readable (for most of us), and you will need a tool to analyze it. My preference is MAT. http://www.eclipse.org/mat/
I think the best way to create .hprof file in Linux process is with jmap command. For example: jmap -dump:format=b,file=filename.hprof {PID}
In addition to using the mentioned jconsole/visualvm, you can use jstack -l <vm-id> on another command line window, and capture that output.
The <vm-id> can be found using the task manager (it is the process id on windows and unix), or using jps.
Both jstack and jps are include in the Sun JDK version 6 and higher.
I recommend the Java VisualVM distributed with the JDK (jvisualvm.exe). It can connect dynamically and access the threads and heap. I have found in invaluable for some problems.
If you are on server-jre 8 and above you can use this:
jcmd PID GC.heap_dump /tmp/dump
Try one of below options.
For 32 bit JVM:
jmap -dump:format=b,file=<heap_dump_filename> <pid>
For 64 bit JVM (explicitly quoting):
jmap -J-d64 -dump:format=b,file=<heap_dump_filename> <pid>
For 64 bit JVM with G1GC algorithm in VM parameters (Only live objects heap is generated with G1GC algorithm):
jmap -J-d64 -dump:live,format=b,file=<heap_dump_filename> <pid>
Related SE question: Java heap dump error with jmap command : Premature EOF
Have a look at various options of jmap at this article
If you want a heapdump on out-of-memory, you can start Java with the option -XX:-HeapDumpOnOutOfMemoryError
You can send the kill -3 <pid> from Cygwin. You have to use the Cygwin ps options to find windows processes then just send the signal to that process.
You could run jconsole (included with Java 6's SDK) then connect to your Java application. It will show you every Thread running and its stack trace.
You have to redirect output from second java executable to some file. Then, use SendSignal to send "-3" to your second process.
The following script uses PsExec to connect to another Windows Session so it works even when connected via Remote Desktop Service.
I wrote a small batch script for Java 8 (using PsExec and jcmd) named jvmdump.bat, which dumps the threads, heap, system properties, and JVM args.
:: set the paths for your environment
set PsExec=C:\Apps\SysInternals\PsExec.exe
set JAVA_HOME=C:\Apps\Java\jdk1.8.0_121
set DUMP_DIR=C:\temp
@echo off
set PID=%1
if "%PID%"=="" (
echo usage: jvmdump.bat {pid}
exit /b
)
for /f "tokens=2,3,4 delims=/ " %%f in ('date /t') do set timestamp_d=%%h%%g%%f
for /f "tokens=1,2 delims=: " %%f in ('time /t') do set timestamp_t=%%f%%g
set timestamp=%timestamp_d%%timestamp_t%
echo datetime is: %timestamp%
echo ### Version >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
%PsExec% -s %JAVA_HOME%\bin\jcmd.exe %PID% VM.version >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo. >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo ### Uptime >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
%PsExec% -s %JAVA_HOME%\bin\jcmd.exe %PID% VM.uptime >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo. >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo ### Command >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
%PsExec% -s %JAVA_HOME%\bin\jcmd.exe %PID% VM.command_line >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo. >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo ### Flags >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
%PsExec% -s %JAVA_HOME%\bin\jcmd.exe %PID% VM.flags >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo. >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
echo ### Properties >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
%PsExec% -s %JAVA_HOME%\bin\jcmd.exe %PID% VM.system_properties >>"%DUMP_DIR%\%PID%-%timestamp%-jvm.log"
%PsExec% -s %JAVA_HOME%\bin\jcmd.exe %PID% Thread.print -l >"%DUMP_DIR%\%PID%-%timestamp%-threads.log"
%PsExec% -s %JAVA_HOME%\bin\jcmd.exe %PID% GC.heap_dump "%DUMP_DIR%\%PID%-%timestamp%-heap.hprof"
echo Dumped to %DUMP_DIR%
It must be run in the same Windows session of the user that started the JVM, so if you connect through Remote Desktop you might need to launch a command prompt in Session 0 and run it from there. e.g.
%PsExec% -s -h -d -i 0 cmd.exe
This will prompt you (click the taskbar icon at the bottom) to View the message in the interactive session, which will take you to the new console in the other session from which you can run the jvmdump.bat script.
How to get process id of java application?
Execute the command 'jcmd' to get the process id of java applications.
How to get Thread dump?
jcmd PID Thread.print > thread.dump
Reference link
You can even use jstack to get thread dump (jstack PID > thread.dump). Reference link
How to get heap dump?
Use jmap tool to get heap dump. jmap -F -dump:live,format=b,file=heap.bin PID
PID stands for process id of the application. Reference link
If you are using JDK 1.6 or above, You can use jmap command to take a heap Dump of a Java process, condition is you should known ProcessID.
If you are on Windows Machine, you can use Task Manager to get PID. For Linux machine you can use varieties of command like ps -A | grep java or netstat -tupln | grep java or top | grep java, depends on your application.
Then you can use the command like jmap -dump:format=b,file=sample_heap_dump.hprof 1234 where 1234 is PID.
There are varieties of tool available to interpret the hprof file. I will recommend Oracle's visualvm tool, which is simple to use.
If you can't (or don't want) to use the console/terminal for some reason, there is an alternative solution. You can make the Java application print the thread dump for you. The code that collects the Stack Trace is reasonable simple and can be attached to a button or a web interface.
private static String getThreadDump() {
Map<Thread, StackTraceElement[]> allStackTraces = Thread.getAllStackTraces();
StringBuilder out = new StringBuilder();
for (Map.Entry<Thread, StackTraceElement[]> entry : allStackTraces.entrySet()) {
Thread thread = entry.getKey();
StackTraceElement[] elements = entry.getValue();
out.append(String.format("%s | prio=%d | %s", thread.getName(), thread.getPriority(), thread.getState()));
out.append('\n');
for (StackTraceElement element : elements) {
out.append(element.toString()).append('\n');
}
out.append('\n');
}
return out.toString();
}
This method will return a string that looks like this:
main | prio=5 | RUNNABLE
java.lang.Thread.dumpThreads(Native Method)
java.lang.Thread.getAllStackTraces(Thread.java:1607)
Main.getThreadDump(Main.java:8)
Main.main(Main.java:36)
Monitor Ctrl-Break | prio=5 | RUNNABLE
java.net.PlainSocketImpl.initProto(Native Method)
java.net.PlainSocketImpl.<clinit>(PlainSocketImpl.java:45)
java.net.Socket.setImpl(Socket.java:503)
java.net.Socket.<init>(Socket.java:424)
java.net.Socket.<init>(Socket.java:211)
com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:59)
Finalizer | prio=8 | WAITING
java.lang.Object.wait(Native Method)
java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)
Reference Handler | prio=10 | WAITING
java.lang.Object.wait(Native Method)
java.lang.Object.wait(Object.java:502)
java.lang.ref.Reference.tryHandlePending(Reference.java:191)
java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
For those interested in a Java 8 version with streams, the code is even more compact:
private static String getThreadDump() {
Map<Thread, StackTraceElement[]> allStackTraces = Thread.getAllStackTraces();
StringBuilder out = new StringBuilder();
allStackTraces.forEach((thread, elements) -> {
out.append(String.format("%s | prio=%d | %s", thread.getName(), thread.getPriority(), thread.getState()));
out.append('\n');
Arrays.stream(elements).forEach(element -> out.append(element.toString()).append('\n'));
out.append('\n');
});
return out.toString();
}
You can easily test this code with:
System.out.print(getThreadDump());
Below java code is used to get the Heap Dump of a Java Process by providing a remote process' PID. The Program uses Remote JMX connection to dump heap to a file. It may be helpful for some one. Doesn't require jmap.
import java.lang.management.ManagementFactory;
import javax.management.MBeanServerConnection;
import javax.management.remote.JMXConnector;
import javax.management.remote.JMXConnectorFactory;
import javax.management.remote.JMXServiceURL;
import java.lang.reflect.Method;
public class HeapDumper {
public static final String HOST = "192.168.11.177";
public static final String PORT = "1600";
public static final String FILE_NAME = "heapDump.hprof";
public static final String FOLDER_PATH = "C:/";
private static final String HOTSPOT_BEAN_NAME ="com.sun.management:type=HotSpotDiagnostic";
public static void main(String[] args) {
if(args.length == 0) {
System.out.println("Enter PID of the Java Process !!!");
return;
}
String pidString = args[0];
int pid = -1;
if(pidString!=null && pidString.length() > 0) {
try {
pid = Integer.parseInt(pidString);
}
catch(Exception e) {
System.out.println("PID is not Valid !!!");
return;
}
}
boolean isHeapDumpSuccess = false;
boolean live = true;
if(pid > 0) {
MBeanServerConnection beanServerConn = getJMXConnection();
if(beanServerConn!=null) {
Class clazz = null;
String dumpFile = FOLDER_PATH+"/"+FILE_NAME;
try{
clazz = Class.forName("com.sun.management.HotSpotDiagnosticMXBean");
Object hotspotMBean = ManagementFactory.newPlatformMXBeanProxy(beanServerConn, HOTSPOT_BEAN_NAME, clazz);
Method method = clazz.getMethod("dumpHeap", new Class[]{String.class , boolean.class});
method.setAccessible(true);
method.invoke(hotspotMBean , new Object[] {dumpFile, new Boolean(live)});
isHeapDumpSuccess = true;
}
catch(Exception e){
e.printStackTrace();
isHeapDumpSuccess = false;
}
finally{
clazz = null;
}
}
}
if(isHeapDumpSuccess){
System.out.println("HeapDump is Success !!!");
}
else{
System.out.println("HeapDump is not Success !!!");
}
}
private static MBeanServerConnection getJMXConnection() {
MBeanServerConnection mbeanServerConnection = null;
String urlString = "service:jmx:rmi:///jndi/rmi://" + HOST + ":" + PORT + "/jmxrmi";
try {
JMXServiceURL url = new JMXServiceURL(urlString);
JMXConnector jmxConnector = JMXConnectorFactory.connect(url);
mbeanServerConnection = jmxConnector.getMBeanServerConnection();
System.out.println("JMX Connection is Success for the URL :"+urlString);
}
catch(Exception e) {
System.out.println("JMX Connection Failed !!!");
}
return mbeanServerConnection;
}
}
Maybe jcmd?
Jcmd utility is used to send diagnostic command requests to the JVM, where these requests are useful for controlling Java Flight Recordings, troubleshoot, and diagnose JVM and Java Applications.
The jcmd tool was introduced with Oracle's Java 7 and is particularly useful in troubleshooting issues with JVM applications by using it to identify Java processes' IDs (akin to jps), acquiring heap dumps (akin to jmap), acquiring thread dumps (akin to jstack), viewing virtual machine characteristics such as system properties and command-line flags (akin to jinfo), and acquiring garbage collection statistics (akin to jstat). The jcmd tool has been called "a swiss-army knife for investigating and resolving issues with your JVM application" and a "hidden gem."
Here’s the process you’ll need to use in invoking the jcmd:
jcmd <pid> GC.heap_dump <file-path> Check it out for more information about taking Java heap dump.
Inorder to take thread dump/heap dump from a child java process in windows, you need to identify the child process Id as first step.
By issuing the command: jps you will be able get all java process Ids that are running on your windows machine. From this list you need to select child process Id. Once you have child process Id, there are various options to capture thread dump and heap dumps.
Capturing Thread Dumps:
There are 8 options to capture thread dumps:
Details about each option can be found in this article. Once you have capture thread dumps, you can use tools like fastThread, Samuraito analyze thread dumps.
Capturing Heap Dumps:
There are 7 options to capture heap dumps:
jmap
-XX:+HeapDumpOnOutOfMemoryError
jcmd
JVisualVM
JMX
Programmatic Approach
Administrative consoles
Details about each option can be found in this article. Once you have captured heap dump, you may use tools like Eclipse Memory Analysis tool, HeapHero to analyze the captured heap dumps.
Visualvm followup:
If you "can't connect" to your running JVM from jvisualvm because you didn't start it with the right JVM arguments (and it's on remote box), run jstatd on the remote box, then, assuming you have a direct connection, add it as a "remote host" in visualvm, double click the host name, and all other JVM's on that box will magically show up in visualvm.
If you don't have "direct connection" to ports on that box, you can also do this through a proxy.
Once you can see the process you want, drill into it in jvisualvm and use monitor tab -> "heapdump" button.
On a Oracle JDK, we have a command called jmap (available in the bin folder of Java Home). usage of the command comes as follows
jmap (option) (pid)
Example: jmap -dump:live,format=b,file=heap.bin (pid)
