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# Runtime Library Support

GWT supports only a small subset of the classes available in the Java 2 Standard and Enterprise Edition libraries, as these libraries are quite large and rely on functionality that is unavailable within web browsers. To find out exactly which classes and methods are supported for core Java runtime packages, see the API reference for `java.lang` and `java.util`, which lists supported classes and contains notes on behavioral differences from the standard Java runtime.

Some specific areas in which GWT emulation differs from the standard Java runtime:

- Regular Expressions
  The syntax of Java regular expressions is similar, but not identical, to JavaScript regular expressions. For example, the replaceAll and split methods use regular expressions. So, you'll probably want to be careful to only use Java regular expressions that have the same meaning in JavaScript.
- Serialization
  Java serialization relies on a few mechanisms that are not available in compiled JavaScript, such as dynamic class loading and reflection. As a result, GWT does not support standard Java serialization. Instead, GWT has an RPC facility that provides automatic object serialization to and from the server for the purpose of invoking remote methods.

Tip
You'll save yourself a lot of frustration if you make sure that you use only translatable classes in your client-side code from the very beginning. To help you identify problems early, your code is checked against the JRE emulation library whenever you run in hosted mode. As a result, most uses of unsupported libraries will be caught the first time you attempt to run your application. So, run early and often.

## Related topics

java.lang, java.util