# Java Applications¶

HTCondor allows users to access a wide variety of machines distributed around the world. The Java Virtual Machine (JVM) provides a uniform platform on any machine, regardless of the machine’s architecture or operating system. The HTCondor Java universe brings together these two features to create a distributed, homogeneous computing environment.

Compiled Java programs can be submitted to HTCondor, and HTCondor can execute the programs on any machine in the pool that will run the Java Virtual Machine.

The condor_status command can be used to see a list of machines in the pool for which HTCondor can use the Java Virtual Machine.

% condor_status -java

Name               JavaVendor Ver    State     Activity LoadAv  Mem  ActvtyTime

adelie01.cs.wisc.e Sun Micros 1.6.0_ Claimed   Busy     0.090   873  0+00:02:46
adelie02.cs.wisc.e Sun Micros 1.6.0_ Owner     Idle     0.210   873  0+03:19:32
slot10@bio.cs.wisc Sun Micros 1.6.0_ Unclaimed Idle     0.000   118  7+03:13:28
slot2@bio.cs.wisc. Sun Micros 1.6.0_ Unclaimed Idle     0.000   118  7+03:13:28
...


If there is no output from the condor_status command, then HTCondor does not know the location details of the Java Virtual Machine on machines in the pool, or no machines have Java correctly installed. In this case, contact your system administrator or see the Java Support Installation section for more information on getting HTCondor to work together with Java.

## A Simple Example Java Application¶

Here is a complete, if simple, example. Start with a simple Java program, Hello.java:

public class Hello {
public static void main( String [] args ) {
System.out.println("Hello, world!\n");
}
}


Build this program using your Java compiler. On most platforms, this is accomplished with the command

javac Hello.java


Submission to HTCondor requires a submit description file. If submitting where files are accessible using a shared file system, this simple submit description file works:

####################
#
# Example 1
# Execute a single Java class
#
####################

universe       = java
executable     = Hello.class
arguments      = Hello
output         = Hello.output
error          = Hello.error
queue


The Java universe must be explicitly selected.

The main class of the program is given in the executable statement. This is a file name which contains the entry point of the program. The name of the main class (not a file name) must be specified as the first argument to the program.

If submitting the job where a shared file system is not accessible, the submit description file becomes:

####################
#
# Example 2
# Execute a single Java class,
# not on a shared file system
#
####################

universe       = java
executable     = Hello.class
arguments      = Hello
output         = Hello.output
error          = Hello.error
should_transfer_files = YES
when_to_transfer_output = ON_EXIT
queue


To submit the job, where the submit description file is named Hello.cmd, execute

condor_submit Hello.cmd


To monitor the job, the commands condor_q and condor_rm are used as with all jobs.

## Less Simple Java Specifications¶

Specifying more than 1 class file.

For programs that consist of more than one .class file, identify the files in the submit description file:

executable = Stooges.class
transfer_input_files = Larry.class,Curly.class,Moe.class


The executable command does not change. It still identifies the class file that contains the program’s entry point.

JAR files.

If the program consists of a large number of class files, it may be easier to collect them all together into a single Java Archive (JAR) file. A JAR can be created with:

% jar cvf Library.jar Larry.class Curly.class Moe.class Stooges.class


HTCondor must then be told where to find the JAR as well as to use the JAR. The JAR file that contains the entry point is specified with the executable command. All JAR files are specified with the jar_files command. For this example that collected all the class files into a single JAR file, the submit description file contains:

executable = Library.jar
jar_files = Library.jar


Note that the JVM must know whether it is receiving JAR files or class files. Therefore, HTCondor must also be informed, in order to pass the information on to the JVM. That is why there is a difference in submit description file commands for the two ways of specifying files (transfer_input_files and jar_files ).

If there are multiple JAR files, the executable command specifies the JAR file that contains the program’s entry point. This file is also listed with the jar_files command:

executable = sortmerge.jar
jar_files = sortmerge.jar,statemap.jar

Using a third-party JAR file.

As HTCondor requires that all JAR files (third-party or not) be available, specification of a third-party JAR file is no different than other JAR files. If the sortmerge example above also relies on version 2.1 from http://jakarta.apache.org/commons/lang/, and this JAR file has been placed in the same directory with the other JAR files, then the submit description file contains

executable = sortmerge.jar
jar_files = sortmerge.jar,statemap.jar,commons-lang-2.1.jar

An executable JAR file.

When the JAR file is an executable, specify the program’s entry point in the arguments command:

executable = anexecutable.jar
jar_files  = anexecutable.jar
arguments  = some.main.ClassFile

Discovering the main class within a JAR file.

As of Java version 1.4, Java virtual machines have a -jar option, which takes a single JAR file as an argument. With this option, the Java virtual machine discovers the main class to run from the contents of the Manifest file, which is bundled within the JAR file. HTCondor’s java universe does not support this discovery, so before submitting the job, the name of the main class must be identified.

For a Java application which is run on the command line with

java -jar OneJarFile.jar


the equivalent version after discovery might look like

java -classpath OneJarFile.jar TheMainClass


The specified value for TheMainClass can be discovered by unjarring the JAR file, and looking for the MainClass definition in the Manifest file. Use that definition in the HTCondor submit description file. Partial contents of that file Java universe submit file will appear as

universe   = java
executable =  OneJarFile.jar
jar_files = OneJarFile.jar
Arguments = TheMainClass More-Arguments
queue

Packages.

An example of a Java class that is declared in a non-default package is

package hpc;

public class CondorDriver
{
// class definition here
}


The JVM needs to know the location of this package. It is passed as a command-line argument, implying the use of the naming convention and directory structure.

Therefore, the submit description file for this example will contain

arguments = hpc.CondorDriver

JVM-version specific features.

If the program uses Java features found only in certain JVMs, then the Java application submitted to HTCondor must only run on those machines within the pool that run the needed JVM. Inform HTCondor by adding a requirements statement to the submit description file. For example, to require version 3.2, add to the submit description file:

requirements = (JavaVersion=="3.2")

Benchmark speeds.

Each machine with Java capability in an HTCondor pool will execute a benchmark to determine its speed. The benchmark is taken when HTCondor is started on the machine, and it uses the SciMark2 (http://math.nist.gov/scimark2) benchmark. The result of the benchmark is held as an attribute within the machine ClassAd. The attribute is called JavaMFlops. Jobs that are run under the Java universe (as all other HTCondor jobs) may prefer or require a machine of a specific speed by setting rank or requirements in the submit description file. As an example, to execute only on machines of a minimum speed:

requirements = (JavaMFlops>4.5)

JVM options.

Options to the JVM itself are specified in the submit description file:

java_vm_args = -DMyProperty=Value -verbose:gc -Xmx1024m


These options are those which go after the java command, but before the user’s main class. Do not use this to set the classpath, as HTCondor handles that itself. Setting these options is useful for setting system properties, system assertions and debugging certain kinds of problems.

## Chirp I/O¶

If a job has more sophisticated I/O requirements that cannot be met by HTCondor’s file transfer mechanism, then the Chirp facility may provide a solution. Chirp has two advantages over simple, whole-file transfers. First, it permits the input files to be decided upon at run-time rather than submit time, and second, it permits partial-file I/O with results than can be seen as the program executes. However, small changes to the program are required in order to take advantage of Chirp. Depending on the style of the program, use either Chirp I/O streams or UNIX-like I/O functions.

Chirp I/O streams are the easiest way to get started. Modify the program to use the objects ChirpInputStream and ChirpOutputStream instead of FileInputStream and FileOutputStream. These classes are completely documented in the HTCondor Software Developer’s Kit (SDK). Here is a simple code example:

import java.io.*;
import edu.wisc.cs.condor.chirp.*;

public class TestChirp {

public static void main( String args[] ) {

try {
new ChirpInputStream("input")));

PrintWriter out = new PrintWriter(
new OutputStreamWriter(
new ChirpOutputStream("output")));

while(true) {
if(line==null) break;
out.println(line);
}
out.close();
} catch( IOException e ) {
System.out.println(e);
}
}
}


To perform UNIX-like I/O with Chirp, create a ChirpClient object. This object supports familiar operations such as open, read, write, and close. Exhaustive detail of the methods may be found in the HTCondor SDK, but here is a brief example:

import java.io.*;
import edu.wisc.cs.condor.chirp.*;

public class TestChirp {

public static void main( String args[] ) {

try {
ChirpClient client = new ChirpClient();
String message = "Hello, world!\n";
byte [] buffer = message.getBytes();

// Note that we should check that actual==length.
// However, skip it for clarity.

int fd = client.open("output","wct",0777);
int actual = client.write(fd,buffer,0,buffer.length);
client.close(fd);

client.rename("output","output.new");

} catch( IOException e ) {
System.out.println(e);
}
}
}


Regardless of which I/O style, the Chirp library must be specified and included with the job. The Chirp JAR (Chirp.jar) is found in the lib directory of the HTCondor installation. Copy it into your working directory in order to compile the program after modification to use Chirp I/O.

% condor_config_val LIB
/usr/local/condor/lib
% cp /usr/local/condor/lib/Chirp.jar .


Rebuild the program with the Chirp JAR file in the class path.

% javac -classpath Chirp.jar:. TestChirp.java


The Chirp JAR file must be specified in the submit description file. Here is an example submit description file that works for both of the given test programs:

universe = java
executable = TestChirp.class
arguments = TestChirp
jar_files = Chirp.jar
+WantIOProxy = True
queue