Using Java 7 to target much older JVMs

Overview

Java 5.0 and 6 used to have poor support for compiling classes to target older versions of Java. It always supported the previous version, but often no more.

Even if you could compile for previous version, you had to be careful not to use functionality which did exist in the previous versions.

Java 7

Java 7 addresses both these issues. Firstly, it supports sources back to 1.2 and targets back to Java 1.1. Secondly. it insists you set the bootclasspath so you can include the version of the libraries you will be using for that version. 

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

$ javac -target 1.7 -source 1.7 Main.java
$ javac -target 1.6 -source 1.6 Main.java
warning: [options] bootstrap class path not set in conjunction with -source 1.6
1 warning
$ javac -Xbootclasspath:/usr/java/jdk1.6.0_29/jre/lib/rt.jar -target 1.6 -source 1.6 Main.java
$ javac -Xbootclasspath:/usr/java/jdk1.5.0_22/jre/lib/rt.jar -target 1.5 -source 1.5 Main.java
$ javac -Xbootclasspath:/usr/java/jdk1.4.0_30/jre/lib/rt.jar -target 1.4 -source 1.4 Main.java
$ javac -Xbootclasspath:/usr/java/jdk1.3.1_29/jre/lib/rt.jar -target 1.3 -source 1.3 Main.java
$ javac -Xbootclasspath:/usr/java/jdk1.2.2_017/jre/lib/rt.jar -target 1.2 -source 1.2 Main.java
$ javac -Xbootclasspath:/usr/java/jdk1.1.8_16/jre/lib/rt.jar -target 1.1 -source 1.2 Main.java
$ javac -Xbootclasspath:/usr/java/jdk1.1.8_16/jre/lib/rt.jar -target 1.1 -source 1.1 Main.java
javac: invalid source release: 1.1
Usage: javac  
use -help for a list of possible options
$ javac -Xbootclasspath:/usr/java/jdk1.1.8_16/jre/lib/rt.jar -target 1.0 -source 1.0 Main.java
javac: invalid target release: 1.0
Usage: javac  
use -help for a list of possible options

BTW: All the previous versions back to 1.1 are available from the Oracle web site Oracle Java Archive

Comments

  1. javin paul10 February 2012 at 16:47

    Its pretty important to know -target and -source switch as there are times when you work in legacy application which is not running on Java5 and your IDE accidentally copied something available to Java5 only.

    Thanks
    What is the problem while using "==" in autoboxing world in Java 5 ?

    ReplyDelete
  2. Unknown26 February 2014 at 07:56

    i really need a clear explanation on how to set a class path as you mentioned

    ReplyDelete
    Replies
    1. Peter Lawrey2 April 2014 at 07:39

      You use the command line specified above, what is your doubt?

      Delete

Post a Comment

Popular posts from this blog

Demystifying Java Object Sizes: Compact Headers, Compressed Oops, and Beyond

Introduction Measuring an object’s size in Java is not straightforward. The platform encourages you to consider references and abstractions rather than raw memory usage. Still, understanding how objects fit into memory can yield significant benefits, especially for high-performance, low-latency systems. Over time, the JVM has introduced optimisations like Compressed Ordinary Object Pointers (Compressed Oops) and, more recently, Compact Object Headers. Each of these can influence how large or small your objects appear. Understanding these factors helps you reason about memory usage more concretely. Measuring Object Sizes In principle, you can estimate an object’s size by creating instances and observing changes in the JVM’s free memory. However, you must neutralise certain factors to get consistent results. For example, turning off TLAB allocation ( -XX:-UseTLAB ) makes memory usage more directly observable. Repeated measurements and median calculations can reduce the im...
Read more

Asking multiple AI to optimise the same code

As different AIs are implemented differently, they don't all provide the same answer, nor do they consistently outperform one another. The best approach is to use multiple AI and pick the one you like best. My goal here is not to declare a winner based on one example, but instead to show the variety of answers you can get with different AI. I asked each AI to Suggest how to implement this more optimally private static String formatOffset(int millis) { String sign = millis < 0 ? "-" : "+"; int saveSecs = Math.abs(millis) / 1000; int hours = saveSecs / 3600; int mins = ((saveSecs / 60) % 60); int secs = (saveSecs % 60); if (secs == 0) { if (mins == 0) { return sign + twoDigitString(hours); } return sign + twoDigitString(hours) + twoDigitString(mins); } return sign + twoDigitString(hours) + twoDigitString(mins) + twoDigitString(secs); } private static String twoDigitString(int value) { ...
Read more

Hands-On Career: The Evolution of a Java Champion

Image
Table of Contents Introduction Superhuman Intelligence Is Already Here ATMs Didn’t Replace Bank Tellers About Me Multidimensional Growth Areas of Career Development Scope of Consideration Roles Where All Areas Are Important The Range of a Founder’s Role How Will AI Change Development? How You Ask the Question Changes the Result Some key terms in understanding how Generative AI works Estimating the Value of AI-Generated Documentation AI and the Reverse Baltimore Phenomenon The Baltimore Phenomenon The Reverse Baltimore Phenomenon Filling a void Brainstorming Ideas Sample Project 2048 Using Prompts as Meta-Programming When AI is useful What Generative AI Can’t Yet Do Human in the Loop Conclusion This article is background material for this talk Lessons learnt from founding my own company, and over 30 years hands-on coding Introduction Unlike most deterministic development tools, Generative AI is a productivity...
Read more