Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window lmdbjava-native-linux-android-armeabi-v7a-0.9.29-1.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.