CODEWITHSUNDEEP
java
shawn
shawn

Reputation: 4223

Can newer JRE versions run Java programs compiled with older JDK versions?

Would I encounter any problems running Java programs and associated libraries compiled in earlier version of Java when using it later version of Java?

I'm compiling using 1.7 whereas some libraries are compiled using 1.6 and running the entire program in a 1.7 JRE?

Upvotes: 102

Views: 116004

Answers (2)

diginoise
diginoise

Reputation: 7630

TL;DR

Java version almanac is the most comprehensive collection of all incompatibilities between all java versions ever released.

Discussion

You are mostly safe and most products and 3rd party libraries will work. However there do exist (not so-) rare binary incompatibilities.

These incompatibilities happen where the classes compiled using older JDK fail to run in the newer JVM due to compatibility breaking changes introduced in the JDK itself (e.g. class removal, class/method visibility reduction, method signature changes or package changes) were introduced after the code using JDK has been written and compiled.

Compatibility tool

Packaged with JDK 9 and onwards, there is jdeprscan tool, which verifies the compatibility of your code against different versions of JDK. It lists no longer available APIs which are called within your code and suggests alternatives(!). You can specify the target JDK version (works for JDK 17, 11, 9, 8, 7 and 6) and it will list incompatibilities against specified target JDK version.

Also consider: JVM Vendor

When thinking of compatibility of your code and upgrading of the toolset and runtime, think of which vendor to choose. Here is a page with a great discussion and details on the JDK/JVM vendors selection

Additional comment in case of libraries:

A reasonable rule of thumb is to use latest stable release version of library for the JRE version your software targets. Obviously you will find many exceptions from this rule, but in general stability of publicly available libraries usually increases with time.

API compatibility and versioning have to be considered when changing versions of dependencies.

Most popular dependencies will have web pages where such information should be available. If not, consider using alternatives.

If you insist on obscure dependency, here is a great answer on how to find out class version. You might need to unzip the JAR file first.

Upvotes: 119

Ewald
Ewald

Reputation: 5751

*** UPDATE ***

Since around 2020, the below answer is sadly no longer accurate. So much has changed in Java over the previous decade and compatibility was sacrificed to move things forward.

While it remains true that most of your code and knowledge transfers to newer versions, there are many exceptions these days, though, in my experience, migrations can sometimes be quite easy.

*** UPDATE ENDS ***

You would not encounter any problems - that's the magic of Java -it's backwards compatible.You can run almost all code from Java 1 on Java 8. There's no reason why Java 6 code won't run on a Java 8 Runtime.

What is interesting, is that for applications written in, let's say, Java 1.4, you even have speed increases when running them on later runtimes. This is because Java is constantly evolving, not just the language known as "Java", but also the JVM (Java virtual machine). I still have source code from more than 10 years ago that still work, as expected in the latest JVM.

If you want to target, let's say, a Java 5 VM, then you can do that with the Java 8 SDK tools. You can ultimately specify which target VM you wish to support, as long as you bear in mind that a version 5 VM might not support all the features a version 8 VM will.

I've just tested code I wrote in Java 5 against the new Java 8 runtime and everything works as expected, so, even though we have a more powerful language and runtime now, we can continue to use our investments of the past. Just that alone makes Java a great development choice for companies.

Upvotes: 103

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