减少项目之间的依赖

Question

I wanted create a project which should minimise the dependencies between the projects. My project structure is below.

com.example.persistence which will contains all the entities and one BaseDao which will contain a save method for saving the entities.

package com.example.pl;

public class BaseDao {

    public  void save(Object obj){
        System.out.println("Saved the object"+obj);
    }

}

com.example.dao will contain the method for saving the entities into the database.The save method substantially calls the BaseDao.save method which is in the persitence layer.

package com.example.dao;

import com.example.pl.BaseDao;

public class EmployeeDao extends BaseDao{

    @Override
    public void save(Object obj) {
        // TODO Auto-generated method stub
        super.save(obj);
    }

}

com.example.service which is a service layer which will access the Dao.Save() method.

Now I wrote a main method in service layer itself to test the dependencies.when I run the main method it gives a below comilation error

package com.example.sl;

import com.example.dao.EmployeeDao;

public class EmployeeService {

    public static void main(String[] args) {
        EmployeeDao dao =new EmployeeDao();
        dao.save(null);
    }

}



The type com.example.pl.BaseDao cannot be resolved. It is indirectly referenced from required .class files

I know I am missing the jar the from the persistence layer in service layer. so Is there any way without adding the dependency of persistence layer in service layer?

Answer 1

It's difficult to answer without knowing how you are organizing the project. So, I will take the liberty to assume that you are using Maven. I will assume that you defined three projects/modules(persitence, dao, service) which will result in persistence.jar, dao.jar, and service.jar.

So, the service module will depend on dao module and dao module will depend on persistence module. The beauty of maven like tool is that it will bring the dependencies transitively. So, the pom.xml(maven artifact - defines project definition) will only declare dependency on dao.

However, just directly answering your Question, you are missing persistence's lib jar from your classpath. Again, it is difficult to answer w/o knowing your project structure.

Answer 2

In some form or fashion, you will have to add the persistence dependency. If you are using a maven multi-module project, this error can sometimes occur, if you attempt to run a maven command in a sub-project, since it will be unable to locate the jars from the other modules. If it's a normal maven project and your dao.jar depends on the persistence.jar, it should be pulling in the persistence.jar into your project for you via the transitive dependency. (Net result, you are getting it in there some way, even if you don't specifically reference it.)

If you aren't using maven or some other dependency management tool, you will need to put the persistence.jar into your classpath/project. The dependency management tool is just doing it for you, if you have setup the dependencies properly and didn't try to have the service one exclude the persistence from the transitive dependencies coming in from the DAO layer. You have objects in your DAO, which are compiled on top of objects in the persistence.jar. It has to have access to those to do the compilation of your project.

Answer 3

From what it sounds like, the way you have it built out...

• dao depends on persistence , and
• service depends on dao .

That should just work. You're not depending directly on persistence in your service layer, but you use a component that does . Barring anything that may have been overlooked by either of us, your dependency chain would not introduce a direct dependency on persistence .

Depending on how you're doing this (Maven, Gradle, Ivy, etc), it may be of benefit to separate these into their own separate modules; that way, the hierarchy is clear and the likelihood of introducing unintended dependencies is very slim.

With that hierarchy, you would ensure that dao has a depdendency on persistence , and that service has a dependency on dao .