Chapter 3. Supporting Technology

Table of Contents

Java Language & APIs
The java Command
Standard Java APIs
Compilation Paths
Code Analysis Tools
YourKit Java Profiler
Legacy Technology
GJ and JSR-14
Other Useful Development Tools

This section is intended as a repository of acquired knowledge with respect to supporting technology used with DrJava. For each tool or library, a section will address questions like:

Java Language & APIs

The Java language and APIs are documented by Sun at The language actually exists in different "editions": Standard Edition (SE), Enterprise Edition (EE), and a collection of Micro Editions (ME). The Standard Edition is DrJava's target platform. See the JDK section of Getting Started for instruction on installing Java.

The language itself is documented by the Java Language Specification, available at This is a good authoritative reference on the language, and any good Java developer should be familiar with it.

The java Command

Java sources are compiled to a collection of class files. These files can then be interpreted by a Java interpreter, which creates a Java virtual machine (JVM) and executes a class's main method. Sun's java is one such interpreter.

Documentation on the java command can be found with Sun's Java tools documentation.

A typical invocation of the interpreter looks like this:

java -cp lib/plt.jar:classes/base edu.rice.cs.MyClass

The -cp option specifies a class path — a collection of files or directories in which the necessary class files are located. This path is searched from left to right; when the code makes reference to an unknown class name, the first matching class file on the class path is used.

Directories of class files may be packaged and compressed into a single jar file for simplified distribution. These are essentially just zip archives of class file collections. In addition, a default main class can be specified for a jar file. Since all necessary classes for running DrJava are bundled into a single jar file, this allows the application to be run with a simplified command syntax:

java -jar drjava.jar

Some other important options:

  • Assertions (Java assert statements) in the code will be ignored when running unless they are explicitly turned on, using the -ea option. The Ant script's run and test targets do this automatically.

  • Java properties can be defined using -Dkey=value options. (These are accessible within the program using System.getProperty("key").)

  • The JVM's heap size is fixed, and the default size is sometimes too small; to explicitly choose the maximum amount of memory the program can use, include the -XmxmegabytesM option.

Standard Java APIs

DrJava is written in version 5 of Java, and makes extensive use of features that are new in that version. It's especially important that DrJava developers be quite comfortable with Java generics. Sun has posted a tutorial by Gilad Bracha that provides a good overview. There is also a a good summary of Java 5 features available in Sun's documentation.

The API specification is another essential reference for developers, available here: A few important packages in the API include:


The Object class, wrapper classes, strings, Iterable (the fundamental list interface), threads, and interface with the system are all defined here.


Standard lists, maps, and other collections are defined here. Note that the newer, non-synchronized classes (like ArrayList) are usually preferred over their older, synchronized counterparts (like Vector).

Provides access to file reading and writing and other file-system operations.


The DrJava GUI is implemented using Swing, the standard Java GUI framework. (Note that many Swing classes and concepts depend on the java.awt package.)


Remote Method Invocation (RMI) provides a simple method-invocation model for inter-process communication. DrJava uses RMI to run the Interactions Pane interpreter in a separate process. Specific RMI documentation is available. Remote interfaces are interfaces that directly extend java.rmi.Remote and whose methods are declared to throw a RemoteException. Instances of these classes can be exported, which creates a server based on the object. Other processes can then create proxy objects implementing the remote interface type (the proxy's methods forward their calls to the server via TCP/IP networking; arguments and return values are serialized). In order for the client processes to locate the server, RMI applications typically register their exported objects with an RMI registry hosted at a well-known port; however, in DrJava (and many other applications) this is not necessary, and the server's information can simply be passed to the client process on startup.

The Java Tutorial provides a collection of small tutorials covering different parts of the APIs.

While new useful classes are included in each major release (such as version 6) of the Java platform, only those classes that will be available on all DrJava users' machines should be used in development. Thus, currently, only APIs available in Java 5 or previously should be used. (Newer classes and methods should be designated with a "since version" tag in the API documentation.)