Restrictions on Long Values with JSNI

The Java long type cannot be represented in JavaScript as a numeric type, so GWT emulates it using an opaque data structure. This means that JSNI methods cannot process a long as a numeric type. The compiler, therefore, disallows, by default, directly accessing a long from JSNI: JSNI methods cannot have long as a parameter type or a return type, and they cannot access a long using a JSNI reference. If you find yourself wanting to pass a long into or out of a JSNI method, here are some options:

1. For numbers that fit into type double, use type double instead of type long.

2. For computations that require the full long semantics, rearrange the code so that the computations happen in Java instead of in JavaScript. That way, they will use the long emulation.

3. For values meant to be passed through unchanged to Java code, wrap the value in a Long. There are no restrictions on type Long with JSNI methods.

4. If you are sure you know what you are doing, you can add the annotation com.google.gwt.core.client.UnsafeNativeLong to the method. The compiler will then allow you to pass a long into and out of JavaScript. It will still be an opaque data type, however, so the only thing you will be able to do with it will be to pass it back to Java.