Develop Reference

Add jar into running environment

I've got a question on how to add a jar file with code.
The situation is that I want to allow the customers to choose themselves, which database should be connected. Therefore, I'd like to give them the opportunity to load a custoom *.jar into the running software (similar to Add external library in eclipse).
Is there a way how I can manage that? I was trying kind of
import System.getProperty("java.io.tmpdir") + "\\dbdriver.jar";
java.io.tmpdir\dbdriver.jar would be the file, where custom jar-library-imports will be stored by my code. But eclipse didn't seem to like it.
Do you have any idea?

You'll need to read up on Classloaders and Reflection in order to get an understanding of how this works.

This is a problem of loading jars at runtime.
Please take a look at the following link which is pretty similar to what you're looking for.
Loading jars at runtime

If this is what you are trying to accomplish in a .java file it is not correct, you cannot reference a jar file directly in a java file, you can just import single classes or a group of classes using wildcard '*'.
The best approach, to my knowledge, here would be to install an ORM library and then decide with the client what his choice for "more than one" RDBMS would be.

Why use link option when import file into eclipse project?

I know that there is the option to have links to source code under your src directory instead of having the source code files directly in your Eclipse project.
When is this case i.e. links for source code is best used?
I always found it more convenient to have the source code inside the Eclipse project

I can think of two possible use cases for this.
The first would be if you want to keep your source and IDE meta-data separate. For example it may be that some developers use Eclipse and some IntelliJ. In this case you would probably only want the source of the project to be stored in SCM, as otherwise, one set of developers are going to have to remove meta-data before importing the project. If they just link to the source, they can maintain there own meta-data for there IDE. Obviously this isn't an issue if everyone uses the same IDE.
The second use case would be dependencies. Say for example your working on two different projects A and B where A depends on B. If your not using a dependency management tool or willing to build and import the Jar from B to A each time you modify it, you could link to the source in B instead.
I'm sure there are plenty of other use cases floating around.

In addition to what Kingamajick mentions, you could have a structure that causes overly long path names.
Windows can only handle so much (256 characters? in older versions), and a deep package structure easily breaks that limit.
So, having your classes in a shallow directory near the top allows you to have your workspaces deeper down, and still leaves some room to wiggle.

Other scenarios; You have source code which is common for several OS:es, but the Eclipse projects are specific for each OS.
You can also create a form of linked resources that are relative to an environment variable. I've used that for situations where the version control system (ClearCase) adds user-specific catalognames.

Add required plugins (dependencies) programmatically to an existing (plugin) project

In eclipse I can add required plug-ins (Dependencies) within UI (see screenshot above). This leads to an entry in the manifest.mf file.
My question is: How can I do this programmatically?
I imagine something like myProject.getManifest().addRequiredPlugin(new PluginImport(...
I already used the Plug-In Selection Spy and copied the used code. The problem is, that internal classes are beeing used there and I don't want to use internal classes. There must be a better way. Thanks in advance.

I don't believe this is possible.
Eclipse uses the manifest to load your plug-in, and probably doesn't read it again after that's done, so editing it would have no effect (since your code doesn't run until after your plug-in is loaded). As such, I doubt an interface has been provided to do this.
What are you trying to accomplish? I don't know of any use cases where trying to do this would be a good idea.

Using serviceloader on android

I am very new to java and android development and to learn I am trying to start with an application to gather statistics and information like munin does. I am trying to be able to load "plugins" in my application. These plugins are already in the application but I don't want to have to invoke them all separately, but be able to iterate over them. I was trying to use serviceloader but could never get the META-INF/services into my apk. So I am wondering if it is possible to use serviceloader on android
Thanks
EDIT: I am asking about java.util.ServiceLoader, I think it should, but I can't figure out how to get my services folder into META-INF on the apk

There is an open bug report against this issue. See https://code.google.com/p/android/issues/detail?id=59658

The META-INF folder is deliberately excluded from the APK by ApkBuilder; the only comment in ApkBuilder.java is "we need to exclude some other folder (like /META-INF)" but there is no other explanation.
Even after adding META-INF with ant, you will still get in trouble if you want to use Proguard, which refuses to replace the content of META-INF/services/* files or rename them (that's another story, the author wants to keep Proguard agnostic).
However, people using maven may want to check https://github.com/pa314159/maven-android-plugin (the branch named "modified"), that tries to solve both issues. It is a fork from the original "android-maven-plugin" I modified one month ago for my own Android projects.
It also provides a patch for Proguard-4.7
Hope this helps, any feedback is welcome.

I've figured out a solution that may work for some situations. Instead of ServiceLoader, I'm using the org.openide.util.Lookup class / library that comes with NetBeans - it is a superset of ServiceLoader. It does not require NetBeans itself and seems to work ok with Eclipse. It is necessary to replace whatever ServiceLoader functionality you are using in your application with Lookup equivalents, and add the org-openide-util-lookup library. Then, you can just do something like this:
Lookup lookup = new ProxyLookup(Lookup.getDefault(),
Lookups.metaInfServices(myClass.getClassLoader(), "services/"));
And move your ServiceLoader files from META-INF/services/ to services/.
Note that, because of the ProxyLookup, this will continue to work on standard Java environments unchanged (i.e., in those cases it will continue to look in META-INF/services).
Here is a link to the documentation for the library: http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/lookup/Lookups.html
UPDATE
After working with this for a couple of days, it seems to function well - I move between environments (standard Java and Android) and it works properly in each location. The primary downside is having to manually copy the files to the /services directory.

It is possible. You may want to check http://developer.android.com/reference/java/util/ServiceLoader.html

ServiceLoader is stuff from the Java language that is not really relevant on Android. I recommend not using it. If you just want to find a list of classes within your .apk to load, there are all kinds of ways to do this -- put in XMl file in res/xml that lists them, use reflection, annotations, etc.

How to create my own java library(API)?

I created a program in Java and I designed it so that methods that I want them to appear (getter methods) in the main, I can call them easily after initiate the class that holds these methods.
The question is that, I need to make this application (that holds the getter methods) to be like an API so that I can give my application for developers to use my functions (the getter methods) if they need them, and only what they need is to add this file (I think the API after is done shown as .jar file).
How can I make it so that I can make my code reusable with other application? It's similar to the .dll, I think.
Thanks a lot ;)

Create a JAR. Then include the JAR. Any classes in that JAR will be available. Just make sure you protect your code if you are giving out an API. Don't expose any methods / properties to the end user that shouldn't be used.
Edit: In response to your comment, make sure you don't include the source when you package the JAR. Only include the class files. That's the best you can really do.

To be useable as an API, your classes should:
Use a unique package (ideally following the convention, i.e. the reverse of a domain you own as prefix). This prevents naming conflicts
Have only those classes and methods public or protected that are intended to be used by others. This makes it easier to use.
Have extensive Javadoc comments.
Be available as a JAR file - ideally separate JARs for binary distribution, source code and javadoc files.

You need to package your application as a jar file. You can use ant jar task to create jar files or you can use the jar command.
For ant tasks look at this link.
For creating it manually look at this link.

Make sure you write and publish javadocs for all your public and protected classes and methods.

To create the jar:
jar cf <jar_name> <sources>

There are several ways you can expose your code. Creating a jar and distributing that may be the easiest as other developers will just have to include your jar. However, if you are talking about "anyone" accessing your code, a web service may make more sense as you can provide access to the data without providing all of the necessary code. You mention providing access to your getters - if you just create a class that has getters, the other developers can use them, but how are they going to be populated? If your application is self contained in that it gets the necessary data and provides the getters, that should work, but if you are talking about providing access to data from your running application, a web service makes more sense as your application can retrieve the data and provide access via publicly accessible methods.
You most likely want to create interfaces as well so developers can code against the interface and you can change the internal workings without impacting them. Any API that will be used by others should be extensively documented as well.

Well, depends on your IDE. I use Netbeans, so I just hit build project, and viola! A jar file is created in my directory specified. Now, that's just for compiling. All anyone has to do is download your .jar file, and if in Netbeans, right click libraries, add jar/folder, and select the downloaded file.

You can also consider:
Include some samples that demonstrate how to use your library
Build your jar using Apache Maven
Put your jar in a public maven repository
Publish a new version of your library as you find/fix bugs
If you want to hide your implementation, you can pack your jar with obfuscation, so that if someone decompiles your classes, the code will be difficult to read