Some of the issues were covered a few days ago in this thread: Java as a scripting language: using the JNI.

Embedding java-script:
1. widely known scripting language
2. engine source code availability
3. easily embeddable
4. fairly extensibile
5. fair performance

Pros of embedding java:
1. well known language
2. no engine source
3. not as easily embeddable
4. very extensible
5. high performance

I can''t speak to python and lua although I''ve heard good things about both of them.