on the fly : Java Glossary

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on the fly
Creating Java byte code while a program is running either by using javac.exe or the deprecated undocuments sun.com.javac.Main or JavaCompiler, or by composing the byte code directly.

You might use the technique for example to compose customised byte code for a particular spreadsheet or regex then let Hotspot turn it into native code, rather than using in interpretive spreadsheet or regex engine that Hotspot has turned into native code. The advantage is speed.

Sample On The Fly Composition, Compile and Execute

Beware! The following example uses an undocumented, deprecated technique calling com.sun.tools.javac. Main. Normally you would use javax.tools. JavaCompiler.
Here is what the dynamically generated Hypotenuse class looks like:

Limitations

You may wonder why in that sample code I wrote the generated java source to disk only to have javac read it back in again, and why javac wrote the class files to disk only to have Class.forName read them back in again. This is because the javac interface I am using is unofficial and kludgy. JavaCompiler in Java version 1.6 lets you bypass the disk i/o.
ANTLR: byte code parser
ASM Java byte code generator
BCEL (Byte Code Engineering Library)
Beanshell
cglib: code generation library
Class.forName
ClassLoader
disassembler
Evaluating expressions on the fly
IBM’s hints on using JavaCompiler
JASM
Jasmin
Java Bytecode Assembler
Java Class file format
Javac.exe
JavaCompiler: Java class to invoke javac.exe
Jenesis
Jeroen Frijters’ presentation : on on-the-fly byte code generation
Jeroen Frijters’ source code : on on-the-fly byte code generation
JikesBT
ObjectWeb ASM
Oolong
scripting languages
Source code analysis: using dynamic compiler invocation
spreadsheet amanuensis
sun.tools.javac.Main
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