Add uses constraints to Export-Package header

[sewe]

Issue #120 requested OSGi headers. Unfortunately, the Export-Package headers lack uses directives, which can be very important to avoid ClassCastExceptions and other wiring-related issue. Here’s an issue I recently came across (see Eclipse Bug 481923 for the real scenario):

• User installs bundle com.google.gson in version X.
• User installs bundle org.example.one, which has an Import-Package on com.google.gson;version="[X,X+1)" and an Export-Package of org.example.one;uses="com.google.gson".
  • Bundle org.example.one is wired to bundle com.google.gson, version X.
• User installs bundle com.google.gson in version Y > X. The wiring of bundle org.example.one remains unchanged due to the version constraint on org.example.one’s Import-Package.
• User installs bundle org.example.two, which has an Import-Package on com.google.gson;version="[X,Y+1)", com.google.gson.annotations;version="[X,Y+1)", and on org.example.one.
  • Bundle org.example.two is wired to package com.google.gson in version X, as the uses constraints of org.example.one force org.example.two to use the same version, and to com.google.gson.annotations in version Y, as it is the newest available.
  • When org.example.two uses com.google.gson, version X, to deserialize JSON, its @SerializedName annotations won’t be honored, as they are loaded with the classloader of com.google.gson, version Y.

You should probably consider using the maven-bundle-plugin, as it can compute uses directives automatically; maintaining them manually is IMHO far to error-prone.

[swankjesse]

Pull requests welcome on this. I don't think any of us understand, or are eager to understand OSGi.

[sewe]

Pull requests welcome on this. I don't think any of us understand, or are eager to understand OSGi.

Fair enough. I’ll try to come up with something that leaves the existing headers unchanged and only adds the uses directives.

Anyway: For you understanding, all that

Export-Package: com.google.gson;uses:="com.google.gson.annotations"

means is “If you import com.google.gson and com.google.gson.annotations, you must do so from the same bundle (so that they will be loaded with the same classloader).”