jar包和class包之间有什么区别？

Question

I know that in java there's two method of packaging one application. One way is extracting all its dependencies's classes and package them with your classes into a large jar. Another way is that copy all its dependency to lib folder, and package them with your own classes into a large jar. What's difference between these two packaging method ?

Thanks

Answer 1

I don't agree that either of these is a standard method. The standard method is to distribute as many .JAR files as necessary, in whatever directory structure is necessary.

Answer 2

book.xml
<book>
<person>
  <first>Kiran</first>
  <last>Pai</last>
  <age>22</age>
</person>
<person>
  <first>Bill</first>
  <last>Gates</last>
  <age>46</age>
</person>
<person>
  <first>Steve</first>
  <last>Jobs</last>
  <age>40</age>
</person>
<person>
  <first>kunal</first>
  <last>kumar</last>
  <age>25</age>
</person>
</book>

create a xml file book.xml
made a jar file book.xml.jar and 
palce it in war/web-inf/lib folder of your project..
 then it will work..

code to parse xml file in same package in java

import java.io.File;
import java.io.IOException;
import java.io.InputStream;

import javax.servlet.http.*;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;




import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;

@SuppressWarnings("serial")
public class XMLParser extends HttpServlet {
    InputStream istream =getClass().getResourceAsStream("/book.xml");
    public void doGet(HttpServletRequest req, HttpServletResponse resp)throws IOException
    {

        DocumentBuilderFactory docBuilderFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder docBuilder = null;
        try {
            docBuilder = docBuilderFactory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
      Document doc = null;
    try {
        doc = docBuilder.parse (istream);
    } catch (SAXException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

        // normalize text representation
        doc.getDocumentElement ().normalize ();
        System.out.println ("Root element of the doc is " + 
             doc.getDocumentElement().getNodeName());


        NodeList listOfPersons = doc.getElementsByTagName("person");
        int totalPersons = listOfPersons.getLength();
        System.out.println("Total no of people : " + totalPersons);

        for(int s=0; s<listOfPersons.getLength() ; s++){


            Node firstPersonNode = listOfPersons.item(s);
            if(firstPersonNode.getNodeType() == Node.ELEMENT_NODE){


                Element firstPersonElement = (Element)firstPersonNode;

                //-------
                NodeList firstNameList = firstPersonElement.getElementsByTagName("first");
                Element firstNameElement = (Element)firstNameList.item(0);

                NodeList textFNList = firstNameElement.getChildNodes();
                System.out.println("First Name : " + 
                       ((Node)textFNList.item(0)).getNodeValue().trim());

                //-------
                NodeList lastNameList = firstPersonElement.getElementsByTagName("last");
                Element lastNameElement = (Element)lastNameList.item(0);

                NodeList textLNList = lastNameElement.getChildNodes();
                System.out.println("Last Name : " + 
                       ((Node)textLNList.item(0)).getNodeValue().trim());

                //----
                NodeList ageList = firstPersonElement.getElementsByTagName("age");
                Element ageElement = (Element)ageList.item(0);

                NodeList textAgeList = ageElement.getChildNodes();
                System.out.println("Age : " + 
                       ((Node)textAgeList.item(0)).getNodeValue().trim());

                //------


            }//end of if clause


        }//end of for loop with s var

    //System.exit (0);

}//end of main


    }

Answer 3

Well actually the official Java way to package an executable jar is to have all its other jar dependencies placed outside it, in a folder that is then declared as being in the classpath (either in the manifest of the executable jar, or as command line arguments when running the executable jar).

There are tools that try to "stuff in" your classpath dependencies inside your jar file. They do this by either unpackaging all jar files (which are really zip files with a different extension) and then repacking all their contents in one jumbo jar. Or they add the dependencies jar as-is to the interior of the executable jar. Maven has a plugin that does both these things, and Eclipse can do them too from the go (when you export your project as a runnable jar).

I prefer the version in which the actual jars are packaged inside your jar. First off it's more proper as file organisation goes, and second you don't run the risk of having two classes with the same fully qualified name which were initially placed in different jars now being in conflict (effectively one will overwrite the other). What's more, from the classpath point of view, having multiple jar files in the classpath, each with their own class files is NOT the same as having all thse classes placed directly in the classpath, so adding your resources inside your executable jar as jars (and not as class files) goes more towards resepcting that.