ext directory : Java Glossary

2007-08-06 by Roedy Green ©1996-2008 Canadian Mind Products
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ext directory

Any jar included in JRE’s ext directory is searched for class files, automatically considered part of the classpath.

Introduction	Downsides
Applets	Links
Automating

Introduction

Simply copying or moving your jar files to the ext directory pointed to by the system property java.ext.dirs =C:\Program Files\java\jre1.6.0_07\lib\ext automatically puts them on the classpath without having to mention them explicitly. This is a great way to prune back an overblown classpath. It is safest to put your jars in all the ext directories:

where to look for ext directories :

in C:\Program Files\java\jre1.6.0_07\lib\ext in the JRE 1.6.0 on your local hard disk.
in J:\Program Files\java\jdk1.6.0_07\jre\lib\ext in the in the JDK 1.6.0 on your local hard disk.
in C:\Program Files\java\jre1.5.0_16\lib\ext in the older JRE 1.5.0 on your local hard disk.
in J:\Program Files\java\jdk1.5.0_16\jre\lib\ext in the in the older JDK 1.5.0 on your local hard disk.

You never know for sure where your javac.exe or java.exe is going to look. Grrr. You can change the location of the ext directory on the command line.

java.exe -Djava.ext.dirs=C:\mylibs mypackage.MyClass

ext dir with Applets

With Applets you normally arrange to preinstall any large jar files. Then you are stuck with the problem of where to them and how to you arrange to get them on the classpath. You can use Java Web Start or you can scatter them and have the user configure a custom classpath or you can use the ext directory. Sometimes you will have so many jar files, you can’t fit them all on the classpath. What you can do then is look at the system property,

System.getProperty( "java.ext.dirs" );

which you can view by running wassup as an application or Applet. Then put the jar in the corresponding directory. e.g.

java.ext.dirs = C:\Program Files\Java\jre1.6.0_07\lib\ext

Any jars jars in the ext directory will automatically be found without putting them on the classpath.

Opera will likely be in jre1.6.0_07\lib\ext
Netscape 4.79 will likely be in Program Files\Netscape\Communicator\Program\Java\classes
IE 5.5 will likely be in WINNT\Java\Packages
Netscape 8.0 will likely be in jre1.6.0_07\lib\ext

There are at least two ext directories:

where additional jars on the path are kept :

in C:\Program Files\java\jre1.6.0_07\lib\ext in the JRE 1.6.0 on your local hard disk.
in J:\Program Files\java\jdk1.6.0_07\jre\lib\ext in the in the JDK 1.6.0 on your local hard disk.
in C:\Program Files\java\jre1.5.0_16\lib\ext in the older JRE 1.5.0 on your local hard disk.
in J:\Program Files\java\jdk1.5.0_16\jre\lib\ext in the in the older JDK 1.5.0 on your local hard disk.

java.exe uses the JRE version and where javac.exe uses the debugging JDK version.

Automating

Sooner or later you will have to reinstall the JDK/JRE and you will lose your ext directories. You can quickly rebuild them if you maintain a bat file like this and run it after every JRE/JDK install. Adjust the file to account for where your ext dirs are and where the jars are you need.

Downsides

The main problems with the extdir technique are:

You can’t deal with two versions of the same jar. With classpath, you can select one or the other version as the need arises.
There are two different ext directories per JVM installed. You need to propagate your jars to all of them. Often you forget when you upgrade your JVM.

classpath
extension

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