Since Apple removed Java 1.5 and Java 1.4 from Snow Leopard, it's not possible to run software that requires older Java versions, or to develop against older Java versions.

In order to restore Java 1.4 and 1.5 to your Snow Leopard install, you'll need to download the old Leopard versions and put them back where they belong in Snow Leopard. This will only work for Intel Macs.

You'll need to use the Terminal in order to install one or both of the older Java versions. To get started, open up the Terminal and type the commands as shown. You don't need to install both versions. The instructions for each are independent.

To install Java 1.5
First get the Java 1.5 archive and expand it.

cd /tmp/
wget http://tedwise.com/files/java.1.5.0-leopard.tar.gz
tar -zxvf java.1.5.0-leopard.tar.gz

Then move it to the standard OS/X Java location.

sudo mv 1.5.0 /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0-leopard

And finally, setup all of the links required for OS/X.

cd /System/Library/Frameworks/JavaVM.framework/Versions/
sudo rm 1.5.0
sudo ln -s 1.5.0-leopard 1.5.0
sudo ln -s 1.5.0-leopard 1.5

To install Java 1.4
Get the Java 1.4 archive and expand it.

cd /tmp/
wget http://tedwise.com/files/java.1.4.2-leopard.tar.gz
tar -zxvf java.1.4.2-leopard.tar.gz

Then move it to the standard OS/X Java location.

sudo mv 1.4.2 /System/Library/Frameworks/JavaVM.framework/Versions/1.4.2-leopard

And setup all of the links required for OS/X.

cd /System/Library/Frameworks/JavaVM.framework/Versions/
sudo ln -s 1.4.2-leopard 1.4.2
sudo ln -s 1.4.2-leopard 1.4

After installation
After you've installed one or both of the Java versions you can change the default Java used by OS/X. This step isn't necessary unless you need to force OS/X to use one of the versions you've just installed.

If you do want to change the default Java version used by OS/X, open up the Java Preferences utility located in /Applications/Utilities.

You can drag and drop Java versions in the bottom pane to change the order in which they are used. The top version is the default version used by OS/X. Make your change in the bottom pane - this is the JRE used when launching Java applications. The top pane selects the JRE used for browser applets.

2. Instructions for an alternative approach are provided by Chris Snyder. This only works for Java 1.5, but it obtains the Java binaries directly from Apple.

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