DrJava requires Java version 5 or later. Note that you will need to have the JDK (not the JRE) installed if you wish to use the debugger or create Javadoc within DrJava.
We recommend downloading and using Oracle's JDK 6 (from http://www.oracle.com/technetwork/java/javase/downloads/index.html) for Solaris, Linux, and Windows. Other users should use the Java virtual machine that comes with their operating system (including Mac OS X).
Mac OS X 10.7 "Lion" does not include a default Java runtime anymore. Therefore, the operating system may prompt you to install Java if you have not installed it before. You can also download Java for OS X Lion from Apple's website, or search for Java downloads for Mac OS X.
DrJava uses two Java Virtual Machines (one for the main program, and one for the Interactions Pane) that use Java's Remote Method Invocation (RMI) to communicate with each other. RMI uses TCP/IP as the default transport mechanism, so you must have those drivers installed. Without TCP/IP, DrJava will not start properly.
Note that there is an incompatibility between Java's Swing GUI library and the Compiz window manager on Linux. We, the developers of DrJava, cannot do anything to fix this problem. We hope that future versions of Java and Compiz will address the incompatibility. In the meantime, we recommend that you disable Compiz if you experience problems. We also suggest that you use the latest versions of Compiz and Java, so you can benefit from possible bug fixes made by Oracle and the Compiz developers. For more information, see http://drjava.org/compiz/.