JavaCompiler : Java Glossary

JavaCompiler

javax.tools.JavaCompiler is a JDK 1..6+ interface that lets your program fire up the Java compiler, (usually javac.exe) and compile Java source, all under your program’s control, in a platform-independent way.

You don’t use exec or sun.tools.javac. Main any more.

Benefits	Learning More
Sample Code	Links
sun.tools.javac.Main

Benefits

The advantages of the JavaCompiler approach are:

The compiler runs in the same JVM as your program. You don’t have the startup overhead of a second JVM.
The complier stays resident. So when you compile more source files, you don’t have to fire it up from scratch again, the way you do with the command line or exec.
You can use it to compile code that exists only in RAM. You can generate it on the fly, compile it, and execute it. This opens up all kinds of new ways of writing code. For example, imagine a spread sheet program that generated a Java program to compute the cells, the compiled it and ran it. It would run just as fast as a hand-written custom program to do those calculations. You can compile Java source that comes to you over a socket.
JavaCompiler will work with any platform, with any Java compiler. exec hooks have to be tuned to the platform.
ANT, JSP, Hibernate etc. will use this hook. They all need to invoke the compiler repeatedly and efficiently. JSP generates Java source from the mixed HTML-Java source it uses. These tools will no longer have to use illegitimate com.sun.javac. Main back door they are forced to use now.

Sample Code

JavaCompiler’s extreme generality and total lack of documentation makes it unusually inscrutable. The following example generates a Java source program on the fly in RAM, (It does not write it to disk.) and uses the default JavaFileManager to generate the class file in the CWD, Current Working Directory.

Note: the Hypotenuse.class file, that is dynamically generated by TestJavaCompiler in the CWD, must be accessible via the classpath to be able to load. For example, if the CWD is E:\myprojects\com\mindprod\example, then E:\myprojects must be on the classpath.

Here is what the dynamically generated Hypotenuse.java file looks like:

sun.tools.javac.Main

In JDK 1.5- you have the deprecated, undocumented sun.tools.javac.Main to fire up the Javac.exe compiler to run in the same JVM and compile under your control.

Learning More

Sun’s Javadoc on JavaCompiler class : available:

on the web at java.sun.com
in the current JDK 1.6.0_17 or in the old JDK 1.5.0_22 on your local Windows J: drive.

Sun’s Javadoc on JavaFileObject class : available:

on the web at java.sun.com
in the current JDK 1.6.0_17 or in the old JDK 1.5.0_22 on your local Windows J: drive.

Sun’s Javadoc on ForwardingJavaFileObject class : available:

on the web at java.sun.com
in the current JDK 1.6.0_17 or in the old JDK 1.5.0_22 on your local Windows J: drive.

Sun’s Javadoc on SimpleJavaFileObject class : available:

on the web at java.sun.com
in the current JDK 1.6.0_17 or in the old JDK 1.5.0_22 on your local Windows J: drive.

Sun’s Javadoc on JavaFileManager class : available:

on the web at java.sun.com
in the current JDK 1.6.0_17 or in the old JDK 1.5.0_22 on your local Windows J: drive.

Sun’s Javadoc on ToolProvider class : available:

on the web at java.sun.com
in the current JDK 1.6.0_17 or in the old JDK 1.5.0_22 on your local Windows J: drive.

Class.forName
IBM’s hints on using JavaCompiler
Javac.exe
JavaLobby sample code
on the fly compose, compile and run
sun.tools.javac.Main

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