The only Java application I ever heard of for this purpose is the JIExplorer. They attempt to offer an open-source alternative to ACDSee what is a rather popular commercial photo viewer. They support thumbnails but with respect to "tagging" this is very dependent of what you mean by that.
- If you want to tag picture areas (such as offered by for example Facebook for tagging people) you are out of luck.
- If you just need to add keywords to pictures then JIExplorer supports this feature.
However, note that the last release of this project was made almost six years ago. Furthermore, I do not think that you will have luck finding much for this purpose for the following reasons:
- Developing desktop applications with Java is a dying business. Java is used mostly for building business web applications. A big share of Java developers would not really know how to create a desktop application anymore and thus the people doing so in their free time is rather small.
- Most people have startet to put their pictures online and manage them on these platforms as for example Instagram or Facebook. Few people doing open-source are spending a lot of time for writing an application that will not be used.
You mention that you require Java to integrate the tool into some other application. Maybe you already have your own user interface? If so, you are maybe looking for too much in one library and should build your application on top of several other libraries. tumbnailator is an excellent tumbnail generator for Java. And binding files to keywords is a rather simple task you could implement individually.