hasemstation.blogg.se - Compiling java into jar

#COMPILING JAVA INTO JAR HOW TO#
#COMPILING JAVA INTO JAR CODE#

compile and test the version-independent source tree on the oldest supported Java version.How do you verify whether you got it right?Īs I said early, a formal description is complex, so here's that rule of thumb I promised: To determine whether your particular layout works, mentally (or actually).

#COMPILING JAVA INTO JAR CODE#

Version-dependent source trees then selectively enhance that code for newer versions. These are not technical requirements nothing stops you from targeting Java 9 and putting half of the code into src /main /java and the other half, or even all of it, into src /main /java - 9, but that only causes confusion.īy sticking to the guidelines, you keep the source tree's layout as simple as possible.Īny human or tool looking into it sees a fully functioning project that targets the required JVM version.

(One addendum to the last point: If you're offering a feature that only works on a newer Java version and can't be steered around, having a class that throws errors stating "Operation not supported before Java X" in the "regular" source tree counts as complete - my recommendation is to not simply leave it out because that would make the project tough to compile.) Sticking to these guidelines, you keep the source tree as simple as possible

The code in that source folder is complete, meaning it can be compiled, tested and deployed as is, without additional files from version-specific source trees like src /main /java - X.

The code for the oldest supported Java version goes into the project's default root directory, for example src /main /java not src /main /java - X.

I propose two guidelines when organizing source code for MR-JARs: More precisely, I'll give you tips on these topics:

#COMPILING JAVA INTO JAR HOW TO#

Now that you know how to create multi-release JARs and how they work, I want to give you some recommendations for how to make the most out of them. These JVMs thus shadow version-unspecific class files with the newest version-specific ones they support. They do that "searching backwards" from their own version, meaning a Java 10 JVM looks for code in META -INF /versions / 10, then META -INF /versions / 9, then the root directory. Newer JVMs, however, first look into META -INF /versions and only if they don't find a class there, into the JAR's root. It is not possible to distinguish versions before 9 JVMs of version 8 and earlier don't know anything about META -INF /versions and simply load the classes from the package structure in the JAR's root.Ĭonsequentially, it is not possible to distinguish between different versions before 9. To create an MR-JAR, use the new jar option -release $.

Java 9 and newer load version-specific class files if they exist, otherwise falling back to version-unspecific ones.

Java 8 and older load version-unspecific class files.

Multi-release JARs (MR-JARs) are specially prepared JARs that contain bytecode for several major Java versions, where.

JVMs will then load the code that was included for their version. Multi-release JARs allow you to create a single JAR that contains bytecode for several Java versions. On the other hand you're dying to use the newest language features and APIs.įrom Java 9 on, multi-releas JARs give you an opportunity to reconcile these opposing forces - at least under some circumstances. It's never easy to decide which Java version to require for your project: On the one hand you want to give users the freedom of choice, so it would be nice to support several major versions, not just the newest one.