Adding external Jar is not smart in case you want to change the project location in filesystem.

The best way is to add the jar to build path so your project will compile if exported:

1. Create a folder called lib in your project folder.

2. copy to this folder all the jar files you need.

3. Refresh your project in eclipse.

4. Select all the jar files, then right click on one of them and select Build Path -> Add to Build Path

Click on File > Import. The Import window will open.Under Select an import source, click on J2EE > App Client JAR file.Click Next.In the Application Client file field, enter the location and name of the application client JAR file that you want to import. You can click the Browse button to select the JAR file from the file system.In the Application Client project field, type a new project name or select an application client project from the drop-down list. If you type a new name in this field, the application client project will be created based on the version of the application client JAR file, and it will use the default location.In the Target runtime drop-down list, select the application server that you want to target for your development. This selection affects the run time settings by modifying the class path entries for the project.If you want to add the new module to an enterprise application project, select the Add project to an EAR check box and then select an existing enterprise application project from the list or create a new one by clicking New.Note: If you type a new enterprise application project name, the enterprise application project will be created in the default location with the lowest compatible J2EE version based on the version of the project being created. If you want to specify a different version or a different location for the enterprise application, you must use the New Enterprise Application Project wizard.Click Finish to import the application client JAR file.

There are several possible reasons, for the question hasn't mentioned the specific failure, and where it has occurred. The following is a list of possible reasons I could think of, but this may not be exhaustive:

• You can import a class, in a different package only if the class is public. The only exception is when you are using the class in the same package. If the class is an inner class marked as private, then you're well and truly out of luck. The Delegator class in question might not be public, and that's why you may be unable to use it. This issue ought to be caught by the compiler.
• The directory structure within the JAR might not match your package import statements in your classes. This might not be necessary, for Eclipse ought to provide possible fixes, but it is better to verify that nevertheless. Again, the compiler should complain if this is the case.
• If the issue is at runtime, then, it is most likely that the JAR is not available in the runtime classpath. You'll need to configure the Runtime configuration, to add the JAR to the runtime classpath. Refer to the Eclipse documentation on run configurations, if you need to know how to change the runtime classpath.

Note:

1. Exporting the build classpath entries would matter to other projects that depend on the pertinent project; unexported entries will have to be re-imported if required in other projects. This would not apply to a run configuration.

Update

Every Java application needs a main(String[] args] method to start execution. This is the entrypoint for the application. From the comment, it appears that the main method is in a different class. If so, the said class ought to be used to start the application. In Eclipse, a "Run configuration" might be used for the class that lacks this entrypoint, resulting in the described error. One can rectify this by creating a new Run configuration for the class with the said entrypoint. This may be done by one of the following:

• editing the existing Run configuration to use the desired Class (the one with the main method). See the above link, in the third bullet point. Edit the value of the class to be launched.
• creating a new Run configuration for the desired Class. Usually, you'll need to traverse to the desired class, and run your application (using the Alt+Shift+X+J shortcut) from the said class.

If you have a module-info.java class inside of your project, you'll sometimes need to specify explicitly inside of the module-info.java class that your project requires the external jar library. If you don't have a module-info.java, what exactly does the error state when you try to import the library? *EDIT: I see now that the library you're trying to import is supposed to be in the class path, so the above case is probably unlikely. If you still have the problem, (I know you posted this ~2 weeks ago), I may be able to help more if you provide the error that comes up when you try to import the library.