Can I request a DOM document with xpath expression from multiple threads safely?

Question

I plan to use the dom4j DOM Document as a static cache in an application where multiprocess threads can request a document. Given the fact that the document itself will never change, is it possible to request it from multiple threads?

I wrote the following code to test it, but I'm not sure if it really proves that the operation is safe?

    package test.concurrent_dom;

    import org.dom4j.Document;
    import org.dom4j.DocumentException;
    import org.dom4j.DocumentHelper;
    import org.dom4j.Element;
    import org.dom4j.Node;

    /**
     * Hello world!
     *
     */
    public class App extends Thread
    {
        private static final String xml = 
            "<Session>"
                + "<child1 attribute1=\"attribute1value\" attribute2=\"attribute2value\">"
                + "ChildText1</child1>"
                + "<child2 attribute1=\"attribute1value\" attribute2=\"attribute2value\">"
                + "ChildText2</child2>" 
                + "<child3 attribute1=\"attribute1value\" attribute2=\"attribute2value\">"
                + "ChildText3</child3>"
            + "</Session>";

        private static Document document;

        private static Element root;

        public static void main( String[] args ) throws DocumentException
        {
            document = DocumentHelper.parseText(xml);
            root = document.getRootElement();

            Thread t1 = new Thread(){
                public void run(){
                    while(true){

                        try {
                            sleep(3);
                        } catch (InterruptedException e) {                  
                            e.printStackTrace();
                        }

                        Node n1 = root.selectSingleNode("/Session/child1");                 
                        if(!n1.getText().equals("ChildText1")){                     
                            System.out.println("WRONG!");
                        }
                    }
                }
            };

            Thread t2 = new Thread(){
                public void run(){
                    while(true){

                        try {
                            sleep(3);
                        } catch (InterruptedException e) {                  
                            e.printStackTrace();
                        }

                        Node n1 = root.selectSingleNode("/Session/child2");                 
                        if(!n1.getText().equals("ChildText2")){                     
                            System.out.println("WRONG!");
                        }
                    }
                }
            };

            Thread t3 = new Thread(){
                public void run(){
                    while(true){

                        try {
                            sleep(3);
                        } catch (InterruptedException e) {                  
                            e.printStackTrace();
                        }

                        Node n1 = root.selectSingleNode("/Session/child3");                 
                        if(!n1.getText().equals("ChildText3")){                     
                            System.out.println("WRONG!");
                        }
                    }
                }
            };

            t1.start();
            t2.start();
            t3.start();
            System.out.println( "Hello World!" );
        }    

    }

Answer 1

http://xerces.apache.org/xerces2-j/faq-dom.html says

. DOM . DOM , .

, , selectSingleNode - DOM. , , .

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, Jaxen - (W3C DOM, XOM, Dom4J, JDOM), .

, W3C DOM . , . 100% , DOM, , dom4j.

: - Jaxen. - Jaxen FAQ

Answer 2

dom4j DOM, , , , .

, runnables ( ) , , . , .

I

• 3- - , .
• - ,

• final AtomicReference<Thread>owner=new AtomicReference<Thread>() ;
  class TestThread
  {
      private String url ;
      private String expected ;
      TestThread(int index) { url = "/Session/child" + i ; expected="ChildText" + i ; }
      public String toString () {return expected;}
      public void run()
      {
          while(true)
          {
              boolean own=owner.compareAndSet(null,this);
              Node n1 = root.selectSingleNode(url);                 
              boolean wrong = !n1.getText().equals(result);
              owner.compareAndSet(this,null);
              if(!own) { System.out.println ( owner.get() + " conflicts " + this ) }
              if(wrong){System.out.println(this + " WRONG!");
          }
      }
  }
  

  }

try{
    while(true){
    Thread t1 = new TestThread(1);
    t1.start();
    Thread t2 = new TestThread(2);
    t2.start();
    Thread t3 = new TestThread(3);
    t3.start();
    }
}
catch(Throwable thr){
    thr.printStackTrace();
}

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