Tuesday Feb 12, 2008

Using HPROF for finding deadlocks

Most of the information here is based on http://java.sun.com/developer/technicalArticles/Programming/HPROF.html. If you are interested in the complete details about HPROF, please visit that site.

To give you little introduction, HPROF is a memory profiling tool which is loaded in the JVM process context when stated as a command-line option. There are tools such as HAT to analyze the output data. Though a subset of the information such as threads-related and monitors etc are available with a jstack dump, HPROF is , however, preferred for a bigger picture that includes memory-related activities such as heap allocation and CPU profiling.

To run HPROF, use either of the following:

java -agentlib:hprof[=options] ToBeProfiledClass


java -Xrunhprof[:options] ToBeProfiledClass

You can get a list of HPROF options by

C:\\Documents and Settings\\Administrator>java -agentlib:hprof=help

     HPROF: Heap and CPU Profiling Agent (JVMTI Demonstration Code)

hprof usage: java -agentlib:hprof=[help]|[<option>=<value>, ...]

Option Name and Value  Description                    Default
---------------------  -----------                    -------
heap=dump|sites|all    heap profiling                 all
cpu=samples|times|old  CPU usage                      off
monitor=y|n            monitor contention             n
format=a|b             text(txt) or binary output     a
file=<file>            write data to file             java.hprof[.txt]
net=<host>:<port>      send data over a socket        off
depth=<size>           stack trace depth              4
interval=<ms>          sample interval in ms          10
cutoff=<value>         output cutoff point            0.0001
lineno=y|n             line number in traces?         y
thread=y|n             thread in traces?              n
doe=y|n                dump on exit?                  y
msa=y|n                Solaris micro state accounting n
force=y|n              force output to <file>         y
verbose=y|n            print messages about dumps     y

Obsolete Options

  - Get sample cpu information every 20 millisec, with a stack depth of 3:
      java -agentlib:hprof=cpu=samples,interval=20,depth=3 classname
  - Get heap usage information based on the allocation sites:
      java -agentlib:hprof=heap=sites classname

  - The option format=b cannot be used with monitor=y.
  - The option format=b cannot be used with cpu=old|times.
  - Use of the -Xrunhprof interface can still be used, e.g.
       java -Xrunhprof:[help]|[<option>=<value>, ...]
    will behave exactly the same as:
       java -agentlib:hprof=[help]|[<option>=<value>, ...]

  - This is demonstration code for the JVMTI interface and use of BCI,
    it is not an official product or formal part of the J2SE.
  - The -Xrunhprof interface will be removed in a future release.
  - The option format=b is considered experimental, this format may change
    in a future release.

You don't really need to compile a java file to use the HPROF. You can use -J option with javac to see the output.

ex. javac -J-agentlib:hprof= ... file.java ( note that there is no space between -J and -agent)

http://java.sun.com/developer/technicalArticles/Programming/HPROF.html very well explains how to gather and analyze the heap dumps and the CPU usage. Therefore, I would cover there how can you use the HPROF to gather information about the threads, deadlocks and monitors etc.

Take a  deadlock situation

I would refer to the code at http://examples.oreilly.com/jenut/Deadlock.java.

Pasting the code from site:

// This example is from _Java Examples in a Nutshell_. (http://www.oreilly.com)
// Copyright (c) 1997 by David Flanagan
// This example is provided WITHOUT ANY WARRANTY either expressed or implied.
// You may study, use, modify, and distribute it for non-commercial purposes.
// For any commercial use, see http://www.davidflanagan.com/javaexamples

 \* This is a demonstration of how NOT to write multi-threaded programs.
 \* It is a program that purposely causes deadlock between two threads that
 \* are both trying to acquire locks for the same two resources.
 \* To avoid this sort of deadlock when locking multiple resources, all threads
 \* should always acquire their locks in the same order.
public class Deadlock {
  public static void main(String[] args) {
    // These are the two resource objects we'll try to get locks for
    final Object resource1 = "resource1";
    final Object resource2 = "resource2";
    // Here's the first thread.  It tries to lock resource1 then resource2
    Thread t1 = new Thread() {
      public void run() {
        // Lock resource 1
        synchronized(resource1) {
          System.out.println("Thread 1: locked resource 1" )  ;

          // Pause for a bit, simulating some file I/O or something.  
          // Basically, we just want to give the other thread a chance to
          // run.  Threads and deadlock are asynchronous things, but we're
          // trying to force deadlock to happen here...
          try { Thread.sleep(50); } catch (InterruptedException e) {}
          // Now wait 'till we can get a lock on resource 2
          synchronized(resource2) {
            System.out.println("Thread 1: locked resource 2" ) ;
    // Here's the second thread.  It tries to lock resource2 then resource1
    Thread t2 = new Thread() {
      public void run() {
        // This thread locks resource 2 right away
        synchronized(resource2) {
          System.out.println("Thread 2: locked resource 2" ) ;

          // Then it pauses, for the same reason as the first thread does
          try { Thread.sleep(50); } catch (InterruptedException e) {}

          // Then it tries to lock resource1.  But wait!  Thread 1 locked
          // resource1, and won't release it 'till it gets a lock on
          // resource2.  This thread holds the lock on resource2, and won't
          // release it 'till it gets resource1.  We're at an impasse. Neither
          // thread can run, and the program freezes up.
          synchronized(resource1) {
            System.out.println("Thread 2: locked resource 1" ) ;
    // Start the two threads. If all goes as planned, deadlock will occur,
    // and the program will never exit.

C:\\Documents and Settings\\Administrator>java -agentlib:hprof=monitor=y Deadlock
Thread 1: locked resource 1
Thread 2: locked resource 2
2008-02-12 23:59:47
Full thread dump Java HotSpot(TM) Client VM (10.0-b19 mixed mode):

"DestroyJavaVM" prio=6 tid=0x00963800 nid=0xe34 waiting on condition [0x00000000..0x0090fd4c]
   java.lang.Thread.State: RUNNABLE

"Thread-1" prio=6 tid=0x0acff400 nid=0xb44 waiting for monitor entry [0x0b09f000..0x0b09fa94]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at Deadlock$2.run(Deadlock.java:56)
        - waiting to lock <0x06c2ea60> (a java.lang.String)
        - locked <0x06c2ea98> (a java.lang.String)

"Thread-0" prio=6 tid=0x0acfe800 nid=0xc20 waiting for monitor entry [0x0b04f000..0x0b04fb14]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at Deadlock$1.run(Deadlock.java:34)
        - waiting to lock <0x06c2ea98> (a java.lang.String)
        - locked <0x06c2ea60> (a java.lang.String)

"Low Memory Detector" daemon prio=6 tid=0x0ace3c00 nid=0xc78 runnable [0x00000000..0x00000000]
   java.lang.Thread.State: RUNNABLE

"CompilerThread0" daemon prio=10 tid=0x0acd4800 nid=0xb9c waiting on condition [0x00000000..0x0af5f640]
   java.lang.Thread.State: RUNNABLE

"HPROF gc_finish watcher" daemon prio=6 tid=0x0acd3000 nid=0x5d0 runnable [0x00000000..0x00000000]
   java.lang.Thread.State: RUNNABLE

"Attach Listener" daemon prio=10 tid=0x009f6c00 nid=0xf58 runnable [0x00000000..0x00000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=10 tid=0x009f2c00 nid=0xd1c waiting on condition [0x00000000..0x00000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=8 tid=0x009e6800 nid=0xdac in Object.wait() [0x0ac1f000..0x0ac1fa94]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x02a90b38> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:116)
        - locked <0x02a90b38> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:132)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

"Reference Handler" daemon prio=10 tid=0x009e2400 nid=0xd0 in Object.wait() [0x0abcf000..0x0abcfb14]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x02a90a40> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:485)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
        - locked <0x02a90a40> (a java.lang.ref.Reference$Lock)

"VM Thread" prio=10 tid=0x009df400 nid=0x328 runnable

"VM Periodic Task Thread" prio=10 tid=0x0acf6c00 nid=0xc38 waiting on condition

JNI global references: 1339

Found one Java-level deadlock:
  waiting to lock monitor 0x009e66ec (object 0x06c2ea60, a java.lang.String),
  which is held by "Thread-0"
  waiting to lock monitor 0x009e5abc (object 0x06c2ea98, a java.lang.String),
  which is held by "Thread-1"

Java stack information for the threads listed above:
        at Deadlock$2.run(Deadlock.java:56)
        - waiting to lock <0x06c2ea60> (a java.lang.String)
        - locked <0x06c2ea98> (a java.lang.String)
        at Deadlock$1.run(Deadlock.java:34)
        - waiting to lock <0x06c2ea98> (a java.lang.String)
        - locked <0x06c2ea60> (a java.lang.String)

Found 1 deadlock.

 def new generation   total 960K, used 214K [0x02a90000, 0x02b90000, 0x02f70000)

  eden space 896K,  23% used [0x02a90000, 0x02ac5890, 0x02b70000)
  from space 64K,   0% used [0x02b70000, 0x02b70000, 0x02b80000)
  to   space 64K,   0% used [0x02b80000, 0x02b80000, 0x02b90000)
 tenured generation   total 4096K, used 0K [0x02f70000, 0x03370000, 0x06a90000)
   the space 4096K,   0% used [0x02f70000, 0x02f70000, 0x02f70200, 0x03370000)
 compacting perm gen  total 12288K, used 1660K [0x06a90000, 0x07690000, 0x0aa900
   the space 12288K,  13% used [0x06a90000, 0x06c2f2a8, 0x06c2f400, 0x07690000)
No shared spaces configured.

Dumping contended monitor usage ... done.

Note: you can either collect the stats at the end of the program or you can use Ctrl-\\ (on Solaris) or by typing Ctrl-Break (on Win32) to dump the conent in between

You can see above that HPROF has detected the deadlock. If you want to analyze the java.hprof.txt you can have a look below

Analyzing the output (java.hprof.txt)

THREAD START (obj=50000138, id = 200003, name="Signal Dispatcher", group= "system";)
THREAD START (obj=50000138, id = 200004, name="Attach Listener", group= "system";)
THREAD START (obj=50000138, id = 200002, name="HPROF gc_finish watcher", group ="system";)
THREAD START (obj=50000138, id = 200001, name="main", group ="main";)
THREAD START (obj=5000015e, id = 200005, name="Thread-0", group ="main";)
THREAD START (obj=5000015e, id = 200006, name="Thread-1", group ="main";)
THREAD END (id = 200001)
THREAD START (obj=50000138, id = 200007, name="DestroyJavaVM", group ="main";)

The threads of our interest are main (id = 200001) , Thread-0 ( id = 200005) and Thread-1 (id = 200006). It shows that the SIGQUIT signal

Explanation of Thread states

R — Runnable
S — Suspended
CW — Condition Wait
MW — Monitor Wait
ZO - Zombie

monitor dump

    THREAD 200001, trace 300000, status: ZO
    THREAD 200002, trace 300000, status: R
    THREAD 200003, trace 300000, status: R
    THREAD 200004, trace 300000, status: R
    THREAD 200005, trace 300029, status: MW
    THREAD 200006, trace 300030, status: MW
    THREAD 200007, trace 300000, status: R
    MONITOR Ljava/lang/String;
    owner: thread 200005, entry count: 1
    waiting to enter: thread 200006
    waiting to be notified:
    MONITOR Ljava/lang/String;
    owner: thread 200006, entry count: 1
    waiting to enter: thread 200005
    waiting to be notified:

The above shows that both Monitors are of type String and first monitor is owned by Thread-0 and Thread-1 is waiting to enter into it. The second monitor shows exactly the opposite.

Monday Feb 11, 2008

Life of a java class from loading to exit

Step 1. Loading of the class. Throws subclasses of LinkageError ex. ClassFormatError, NoClassDefFoundError.

Step 2. Linking of the class:
The process of taking a binary form of a class or interface type and combining it into the runtime state of the Java virtual machine,
    a. Verification – instruction code is verified.
    b. Preparation – static variables and method tables are created .
    c. Resolution – Checking symbolic references from loaded class to other classes. Resolution translates the names into explicit references.Also checks for  field/method existence and whether access is allowed.

 Step 3. Initialization of the static variables and initializers.Note that the static variables have already been created during the Preparation phase.

Step 4. main(..) is executed.

Step 5. A class or interface may be unloaded if and only if its class loader is unreachable.

Step 6. The Java virtual machine terminates all its activity and exits when one of two things happens:
 - All the threads that are not daemon threads terminate.
-  Some thread invokes the exit method of class Runtime or class System, and the exit operation is permitted by the security manager.

Abstract class vs. Interface

At design-level, an interface describes the functionalites which could be implemented differently across various implementations. Therefore, it de-couples an architecture in terms of the functionalities. Whereas, an abstract class talks about the responsibilites of the classes under a hierarchy. It creates an inheritance hierarchy where subclasses provide
their own definitions for the inherited methods.

As an example, I might decide to write an interface called Accessor to access a data source.I would design GenericAccessor as an abstract class which implements Accessor. GenericAccessor could implement multiple interfaces to implement a range of functionalities which have been divided across multiple interfaces. So it means that GenericAccessor could be an Accessor and a Listener at the same time. If I were to design GenericAccessor as an abstract class, I would be putting all the functionalities inside the single class which
doesn't me incorporate re-usability into the design.

A few interesting facts:

only final static variables are allowed to be declared.
all the declared methods are public
Abstract classes:
both instance and static variables are allowed
can have any kind of methods.

Friday Jun 22, 2007

Some information on the Class Loaders

It's just a recap of the class loaders from my experience.

Class Loaders use a delegation model to delegate the loading of a class to its parent class loaders recursively before handling it.

Arrays don't have any class loaders. The class loader of an array is same as of the element type. Therefore, an array of primitive values doesn't have any class loader.

a. Bootstrap class loader is a native implementation in the JVM and it doesn't have any parent. If you do a getClassLoader() on the class loaded by this  loader, it would return NULL.

 ex. String.class.getClassLoader() = null.< /FONT >

b. ClassLoader for extension (JRE/lib/ext) - sun.misc.Launcher$ExtClassLoader is used for loading the above classes. In some implementations, it is
                combined with bootstrap class loader.
c. Delegation model
 bootstrap <- ExtClassLoader <- AppClassLoader

d. BootstrapClass loader loads the classes from the local file system in a platform-dependent manner. It loads  classes of java.\* ,javax.\* and org.omg.\*, and if the search is made on those loaded classes to find out the class loaders, it would return NULL.

How does a ClassLoader provide security ?

Class loaders play a critical role in providing security because the class loader is responsible for locating and fetching the class file, consulting the security policy and defining the class object with the appropriate permissions.

What is the algorithm for loading a class ?

   a. Check if the class is loaded in the cache.
   b. If the current class loader has a delegation parent , redirect the loading request to that. Otherwise, redirect it to the bootstrap class loader.
   c.  If not found, call a customized method to find the class from the current classloader.

   In pseudo code

What are the rules for loading classes in terms of individual class loaders?

    a. First class of an application: AppClassLoader
    b. First class of an applet: AppletClassLoader
    c. class.forName("someclass") : bootstrap classLoader
   d. A reference inside a loaded class: current class loader

 a delegation model

   ClassLoader::loadClassInternal() -> Called by JVM

What all does a classloader do?

     a.  produces a stream of binary data.
     b. parse the binary data into internal structures in the method area of JVM.
     c. creates an instance of Class to represent the type.



This is the blog of a software engineer, specialized in identity management. Kunal Sinha works in Directory Services Engineering (OpenDS) team from Austin,Texas.


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