Why don't I see code when viewing included Jar libraries in Eclipse?

Question

There doesn't seem to be any code there. I expected to see class declarations so I could see what the code does but instead there's some

Are they somehow precompiled? What's the difference between included Jar file and a pure code?

Answer 1

You are looking at .class file, which is a generated when you compile .java file. To see what the program does, you have to look into .java file. You can refer official java documentation for that.

A .jar file is packaged file with .jar extension, it contains compiled java files and their class files. This file is usually imported into projects to use the classes defined in that package.

You can use "jar xf jar-file" command in command-prompt/terminal to extract the files from jar and look into the package.

Answer 2

A JAR will normally contain compiled class files. It may also contain source files or there may be a separate JAR that contains the source files, but not necessarily so.

If you want to use the library in your project, then a JAR of compiled class files is what you want. If you want the source code, then you'll have to see if it is available from wherever you downloaded this from. If all you want is to see how to use the classes, then probably what you want are JavaDocs for the library you are using. This is an HTML based API documentation.

Answer 3

Well, this is because you haven't attached any source for the mentioned dnsns.jar . You can attach source to existing JAR files in Eclipse. Refer this SO post: Is there an easy way to attach source in Eclipse?

For this specific dnsns.jar , it is part of your JRE, and if you are not able to see its source in your IDE, then it means that the Java that you have setup in IDE lacks the source. If your installation does not have the source ( src.zip ), then you can get it manually as mentioned on this SO post: Where to find Java JDK Source Code?

EDIT: Alternatively, you can also use a decompiler (eg http://jd.benow.ca/ ) to reverse engineer the source from byte code, though, it may not be the exact match to the original source but you can understand the overall idea. You can add the decompiler as the default program for opening .class files in eclipse Windows > Preferences > General > Editors > File Associations . Select *.class filter and add your decompiler as the program. Though, it is not as clean as attaching the source to JAR, but may work if you don't have access to source.

EDIT2: About your question

What's the difference between included Jar file and a pure code

Eclipse can find .java files for your own code because obviously they are in your workspace. But when you add a JAR file as library, it may have the source (.java) in it or not. If the source is available, eclipse can display it by default. If not, you have to add it manually.