I would be very surprised if a JDK1.4 built JAR failed to work in a Java5 environment.
Please note that current Tamino Servers only include a Java 1.4 runtime environment. This means that you would not be able to use Java5 specific JARs from within Tamino (e.g. in server extensions).
However I would expect Java5 built applications using the API to access Tamino to work.
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