You must use a Java 1.5 runtime to use GWT Hosted Mode on Mac OS X.

After the upgrade to Snow Leopard Mac OS X 10.6 I got the following error in GWT Eclipse console:

You must use a Java 1.5 runtime to use GWT Hosted Mode on Mac OS X.

I FIXED this issue by restoring Java 1.5 from my Time Machine in following steps:

1) Verify that Java 1.5.0 is missing from Snow Leopard

1.5.0 -> CurrentJDK

CurrentJDK -> 1.6

uki@Uki:/System/Library/Frameworks/JavaVM.framework/Versions $ ls -alt

total 48

drwxr-xr-x 11 root wheel 374 Sep 19 00:18 .

drwxr-xr-x 11 root wheel 374 Sep 19 00:18 ..

drwxr-xr-x 8 root wheel 272 Sep 19 00:17 A

lrwxr-xr-x 1 root wheel 1 Sep 19 00:17 Current -> A

lrwxr-xr-x 1 root wheel 5 Sep 19 00:17 1.6 -> 1.6.0

drwxr-xr-x 7 root wheel 238 Sep 19 00:17 1.6.0

lrwxr-xr-x 1 root wheel 3 Sep 19 00:17 CurrentJDK -> 1.6

lrwxr-xr-x 1 root wheel 5 Sep 19 00:17 1.3 -> 1.3.1

lrwxr-xr-x 1 root wheel 10 Sep 19 00:17 1.5 -> CurrentJDK

lrwxr-xr-x 1 root wheel 10 Sep 19 00:17 1.5.0 -> CurrentJDK

drwxr-xr-x 3 root wheel 102 Jul 20 18:35 1.3.1

uki@Uki:/System/Library/Frameworks/JavaVM.framework/Versions $

2) Restore Java 1.5.0 from your Time Machine (if you have no backup, shame on you!)

3) Create a symbolic link (sudo ln -s 1.5.0/ CurrentJDK)

uki@Uki:/System/Library/Frameworks/JavaVM.framework/Versions $ sudo ln -s 1.5.0/ CurrentJDK

WARNING: Improper use of the sudo command could lead to data loss

or the deletion of important system files. Please double-check your

typing when using sudo. Type "man sudo" for more information.

To proceed, enter your password, or type Ctrl-C to abort.

Password:

uki@Uki:/System/Library/Frameworks/JavaVM.framework/Versions $ ls -alt

total 40

drwxr-xr-x 11 root wheel 374 Sep 19 06:38 .

lrwxr-xr-x 1 root wheel 6 Sep 19 06:38 CurrentJDK -> 1.5.0/

drwxr-xr-x 11 root wheel 374 Sep 19 00:18 ..

drwxr-xr-x 8 root wheel 272 Sep 19 00:17 A

lrwxr-xr-x 1 root wheel 1 Sep 19 00:17 Current -> A

lrwxr-xr-x 1 root wheel 5 Sep 19 00:17 1.6 -> 1.6.0

drwxr-xr-x 7 root wheel 238 Sep 19 00:17 1.6.0

lrwxr-xr-x 1 root wheel 5 Sep 19 00:17 1.3 -> 1.3.1

lrwxr-xr-x 1 root wheel 10 Sep 19 00:17 1.5 -> CurrentJDK

drwxr-xr-x 3 root wheel 102 Jul 20 18:35 1.3.1

drwxr-xr-x 8 root wheel 272 May 7 2008 1.5.0

uki@Uki:/System/Library/Frameworks/JavaVM.framework/Versions $

4) Verify that you have the right set up

uki@Uki:/System/Library/Frameworks/JavaVM.framework/Versions $ java -version

java version "1.5.0_20"

Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_20-b02-315)

Java HotSpot(TM) Client VM (build 1.5.0_20-141, mixed mode, sharing)

uki@Uki:/System/Library/Frameworks/JavaVM.framework/Versions $

The GWT will work now, I will switch to Java 1.6 AFTER GWT release next update.