How to fix sealing violation from a library? sealing violation: package org.lwjgl.opengl is sealed

Question

While setting up a project as a template for Slick2d based projects following the instructions here:Slick2d wiki using the provided code for testing setup here at run-time I keep getting a giant block of sealing errors. My thought is that this problem stems from the version of ljgwl.jar in both libraries, however Slick requires both in order to function properly. How can I resolve this?

Answer 1

Package sealing is a Java feature implemented in part in the JAR file format. It is discussed in several places, including in Oracle's Java Tutorial, but the bottom line is that when package sealing is enabled in a Jar's manifest, no classes belonging to that package can appear in any other JAR file.

My thought is that this problem stems from the version of ljgwl.jar in both libraries, however Slick requires both in order to function properly.

I'm not sure what you mean by "library", as that is not a Java concept. I suspect, however, that you're trying to say that you somehow have ljgwl.jar files from two separate sources, and you've put both into your project classpath. That would indeed be a problem, and more than just for package sealing. You can, in fact, be thankful for the sealing errors, for they may have saved you from subtler, more difficult to diagnose runtime errors.

How can I resolve this?

You should have only one copy of LWJGL in your classpath, regardless of any requirements enforced by package sealing, and regardless of how the classes are packaged in jar files. It looks like the Slick2D distribution may come with a copy of LWJGL -- in that case, it's probably wisest to use that one. As long as it's in your classpath (as it must be anyway for Slick2D to use it), any class anywhere in your application can use it.

It gets tricky if you need to contend with a inconsistent requirements for LWJGL version, or if you have obtained a JAR that incorporates the LWJGL classes along with something else, but that doesn't change the bottom line: you must choose one version of LWJGL, use that version exclusively within your application, and include only one copy of it in your classpath.