Java: Understanding JSR14
During the development of the Java generics specification (and other language features added in Java 5), experimental support was added to the javac compiler to allow it to consume Java 5 language features and generate bytecode that could be run on a Java 1.4 JVM. While these features are not supported (or even documented), they are used by a number of open source projects to allow developers to code using Java 5 language features and produce JAR files that can be used on earlier JVMs. And, now that javac is open source, it is possible the features might be supported by a third party. To activate these features, you can invoke javac with the -source 1.5 and -target jsr14 options.
The JSR 14 target mode of javac causes the compiler to emit JDK 1.4-compatible bytecode corresponding to Java 5 language features:
- Generics and varargs: The casts inserted by the compiler in the presence of generics have no dependency on the class library, and so they can execute equally well on a pre-5 JVM. Similarly, the code generated by the compiler in the presence of variable-length argument lists has no dependency on the class library.
for-eachloop: When iterating over an array, the compiler generates an induction variable and the standard array iteration idiom. When iterating over aCollection, the compiler generates the standard iterator-based idiom. When iterating over a non-CollectionIterable, the compiler produces an error.- Autoboxing: Rather than generating calls to the
valueOf()method in the wrapper class, the compiler generates calls to the constructor instead. - String concatenation: The JSR 14 target mode of
javaccauses the compiler to generate calls toStringBufferinstead ofStringBuilder. - Enumerations: The JSR 14 target mode of
javachas no special support for enumerations. Code that attempts to use enumerations will fail with aNoClassDefFoundErrorlooking for thejava.lang.Enumbase class.
Using the JSR 14 target mode allows you to write code that uses generics, autoboxing, and the for-each loop in the “easy” cases, which may suffice for many projects. It is convenient, if unsupported, and the compiler generates mostly compatible bytecode in a single pass.
Quoted from
http://www.ibm.com/developerworks/java/library/j-jtp02277.html
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