Weblog Chris Hegarty : Weblog

Based on a peabody contribution by Matthias File.deleteOnExit was re-implemented in mustang b63 and now uses java level shutdown hooks. Previously there was a native implementation that registered filenames to be deleted at exit in a native linked list type structure and which was called during the vm exit to do the cleanup. Diagnosing issues where excessive use of deleteOnExit was causing OutOfMemoryErrors was very difficult as the list was held in native heap.

The re-implementation of deleteOnExit using java level shutdown hooks has several benefits:

• A file name is only added once to the list of files to be deleted, so if deleteOnExit is invoked multiple times on the same File then it will not consume any additional memory. This issue was tracked by 4813777 in Sun bug database.

• The new implementation stores the file names in a HashSet in the java heap. This allows for better diagnosability of further issues with deleteOnExit (and I will describe how to do this later).

There is still an issue with some code that creates a lot of temporary files and calls deleteOnExit on them to ensure that they will be removed during shutdown. This is currently a limitation of the way deleteOnExit is specified as you need to ensure that a potentially different file with the same abstract path is removed even if you specifically call delete on the current one. You need to be careful when using deleteOnExit with temporary files in this manner. So what do you do if you think you are running into this issue, read on...

As I mentioned earlier, in the past it was difficult to figure out if deleteOnExit was causing your OutOfMemoryError's because it allocates in the native heap and not the java heap, and inspecting the native heap is much more difficult. Here are 2 solutions if you are running with mustang b63 (or later) and think you are experiencing issues cause by deleteOnExit.

1. Examine the heap dump

   You can use jmap to dump the java heap of a running process or add -XX:+HeapDumpOnOutOfMemoryError option to your startup arguments. You can see how to do this in Alans blog. Once you have your dump, run jhat on it. If you are not familiar with jhat then look at Sundar's blog. JHat will read the dump file and start a HTTP server. You will see something like this:

   
       Reading from heap.bin...
       Dump file created Wed Feb 01 13:57:42 GMT 2006<
       Snapshot read, resolving...
       Resolving 18956 objects...
       Chasing references, expect 3 dots...
       Eliminating duplicate references...
       Snapshot resolved.
       Started HTTP server on port 7000
       Server is ready.
   

   
   Now connect to the HTTP server using a browser and follow the links, e.g.

   select All classes including platform
   search for class java.io.DeleteOnExitHook, select it
   From its Static Data Members you will see files (L) : java.util.HashSet@0xXXXXXXXX (XX bytes), select it
   In its Instance data members you will see map (L) : java.util.HashMap@0xXXXXXXXX (XX bytes), select it
   Its Instance data members will look something like this:

     entrySet (L) : 
     keySet (L) : 
     loadFactor (F) : 0.75
     modCount (I) : 5000
     size (I) : 5000
     table (L) : [Ljava.util.HashMap$Entry;@0xbbc19ff8 (32776 bytes)
     threshold (I) : 6144
     values (L) : 
   

   size is the number of files currently marked as deleteOnExit. In this example 5000. You can see the amount of heap space they are comsuming by looking at the number of bytes the table instance data member is occuping. In this example 32776 bytes.

   You can also see the files that are currently marked as deleteOnExit by clicking on the table instance data member, which is a HashMap$Entry array. Each HashMap$Entry has a key which is a java.lang.String whose value is the abstract path name.



2. Write yourself a tool

   This is a little diagnostic tool that allows you to poll the length of the deleteOnExit list. It uses reflection to access the same data members as in the above solution, but automates it so you don't need to know about the internal data structures.

   The new Attach API lets you load an agent into a running vm. The following sample code contains a simple agent that uses reflection to find out the number of files marked deleteOnExit and sends this to the client over a Socket. This allows you to connect to an already running java process (started as usual without any special arguments) and obtain a count of the files marked deleteOnExit. You can use jps to retrieve the process id of the java process that you wish to attach to and then pass it as a command line argument to CountDeleteOnExitFiles

   
       weetabix : jps -l
       6829 DeleteOnExitTest
       6835 sun.tools.jps.Jps
       
       weetabix : java -cp /j2se/jdk6.0/lib/tools.jar:. CountDeleteOnExitFiles 6829
       Number of files registered to be deleted on exit is 32
       Number of files registered to be deleted on exit is 37
       Number of files registered to be deleted on exit is 42
       ....
       

   Client that attachs to the target vm and loads the Agent:
   CountDeleteOnExitFiles.java

   
   import java.net.*;
   import java.io.*;
   import com.sun.tools.attach.*;
   
   public class CountDeleteOnExitFiles
   {
       public static void main(String[] args) {
   	if (args.length != 1 || args[0].equals("-h")) {
   	    System.err.println("Prints the number of files registered to be deleted on exit in process <pid> every 5 seconds");
   	    System.err.println("Usage: \n  java CountDeleteOnExitFiles <pid>");
   	    return;
   	}
   
   	try {     
   	    ServerSocket ss = new ServerSocket(0);
   	    int port = ss.getLocalPort();
   	    
   	    // attach to target VM and load Agent
               VirtualMachine vm = VirtualMachine.attach(args[0]);
               
               File jarfile = new File("DeleteOnExitAgent.jar");
               if (!jarfile.exists()) {
                   System.err.println("agent not found");
                   return;
               }
               String agent = jarfile.getCanonicalPath(); 
   
               vm.loadAgent(agent, Integer.toString(port));
               vm.detach();
               
               Socket socket = ss.accept();
   	    ss.close();
   
   	    DataInputStream in = new DataInputStream(socket.getInputStream());
   	    for (;;) {
   		int size = in.readInt();
   		System.out.println("Number of files registered to be deleted on exit is " + size);
   	    }
   	} catch (Exception e) {
   	    e.printStackTrace();
   	    System.exit(0);
   	}
       }
   }
   

   Agent that is loaded into the target vm and sends the count of files marked deleteOnExit to the client (agent uses reflection to access Sun's internal deleteOnExit implementation and is therefore Sun jdk specific). This Agent needs to be deployed as a jar file in accordance with the "Starting Agents After VM Startup" section of java.lang.instrument. Specifically it needs to be compiled and put in a jar file called DeleteOnExitAgent.jar with the attribute "Agent-Class: DeleteOnExitAgent" in its manifest.

     
   weetabix : javac DeleteOnExitAgent.java
   weetabix : more manifest.mf
   Agent-Class: DeleteOnExitAgent
   weetabix : jar -cvfm DeleteOnExitAgent.jar manifest.mf DeleteOnExitAgent*.class
   added manifest
   adding: DeleteOnExitAgent$1.class(in = 486) (out= 338)(deflated 30%)
   adding: DeleteOnExitAgent.class(in = 2357) (out= 1381)(deflated 41%)
   weetabix : cp DeleteOnExitAgent.jar /j2se/jdk6.0/jre/lib/ext
   

   DeleteOnExitAgent.java

   
   import java.util.*;
   import java.io.*;
   import java.net.*;
   import java.lang.reflect.*;
   
   public class DeleteOnExitAgent
   {
       static int DEFAULT_INTERVAL = 5000;
       Socket socket = null;
   
       // invoked by attach mechanism
       public static void agentmain(String agentArgs) {
   	//port number to send data to
   	final int port;
   
   	if (agentArgs != null && agentArgs.equals("")) {
               try {
                   port = Integer.parseInt(agentArgs);
               } catch (NumberFormatException nfe)
                   return;
           }
           else
               return;
   
   	Thread agentThread = new Thread( new Runnable() {
   		public void run() {
    		    startAgent(port);
   		}});
   	agentThread.setDaemon(true);
   	agentThread.start();
       }
   
       static void startAgent(int port) {
   	Socket socket = null;
   	DataOutputStream dataOut = null;
   	try {
   	    socket = new Socket("localhost", port);
   	    dataOut = new DataOutputStream(socket.getOutputStream());
   
   	    Class DeleteOnExitHookClass = Class.forName("java.io.DeleteOnExitHook");
   	    Field filesField = DeleteOnExitHookClass.getDeclaredField("files");
   	    filesField.setAccessible(true);
   	    HashSet hashSet = (HashSet)filesField.get(null);
   
   	    Class hashSetClass = Class.forName("java.util.HashSet");
   	    Field mapField = hashSetClass.getDeclaredField("map");
   	    mapField.setAccessible(true);
   	    HashMap hashMap = (HashMap)mapField.get(hashSet);
       
   	    Class hashMapClass = Class.forName("java.util.HashMap");
   	    Field sizeField = hashMapClass.getDeclaredField("size");
   	    sizeField.setAccessible(true);
   
   	    for (;;) {
   	        int size = sizeField.getInt(hashMap);
   	        dataOut.writeInt(size);
   		Thread.sleep(DEFAULT_INTERVAL);
   	    }
   	} catch (SocketException se) {
   	    try { if (socket != null ) { dataOut.close(); socket.close(); } }
   	    catch (IOException e) { e.printStackTrace(); }
   	} catch (Exception e) {
   	   e.printStackTrace(); 
   	}
       }
   }