Starting a worker thread in a Servlet Engine

Today's example shows how to create a background thread when your servlet context starts up, and how to end the thread's activities gracefully when the servlet context is destroyed. Take this example and add it as a "listener" in your web application, then using the Tomcat (or equivalent) manager start and stop the context and watch the stdout messages.

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class Deamon implements ServletContextListener, Runnable {

   private Thread t;

   public void contextDestroyed(ServletContextEvent arg0) {
      t.interrupt();}

   public void contextInitialized(ServletContextEvent context) {
      t = new Thread(this);
      t.start();
      System.out.println("deamon started");}

   public void run() {
      try {
         while (true) {
            System.out.println("deamon working...");
            Thread.sleep(1000);
         }
      } catch (InterruptedException e) {
         System.out.println("deamon stopped");}
   }
}

All purpose String trimmer

The java string class supports a trim() operation that trims ' '. You may find that you want to trim another character. This terse example will trim any character from the beginning and end of a String. It is based on the JDK's trim() method with a few more attempts at brevity.

   public static String trim(String source, char c) {
      int len = source.length();
      int st = 0;
      char[] val = source.toCharArray();
      for (;(st < len) && (val[st] <= c);st++){}
      for (;(st < len) && (val[len-1]<=c);len--){}
      return ((st > 0) || (len < source.length())) ? 
         source.substring(st, len) : source;
   }

XMP Dumper reduced

I was able to trim this one down a bit more.

import java.io.*;

public class XMPDumper {
  
   public String getXMP(File f) throws IOException {
      return getXMP(new FileInputStream(f));
   }
 
   public String getXMP(InputStream is)
      throws IOException {
     for (int i=0;(i=is.read())!=-1;)
       if (i==0xFF && is.read()==0xE1) {
         String result = getXMPSegment(is);
         if (result.startsWith("http://ns.adobe.com/xap/1.0/"))
            return result.substring(29);
       }
     return null;
   }

   private String getXMPSegment(InputStream is)
      throws IOException {
      DataInputStream dis = new DataInputStream(is);
      byte[] buffer = new byte[dis.readShort()-2];
      dis.read(buffer);
      return new String(buffer);
   }
}

Technorati Profile

XMP metadata dumper

XMP is a metatdata format from Adobe. One way to add and edit XMP is with PixVue. This is a minimalist XMP extractor/dumper for the jpeg format.

import java.io.*;

public class XMPDumper {
  
   public static void main(String[] args) throws IOException {
      File f = new File("myimage.jpg");
      String xmp = new XMPDumper().getXMP(f);
      System.out.println(xmp);
   }

   public String getXMP(File f) 
      throws IOException {
     FileInputStream is = new FileInputStream(f);
     int i=0; String result=null;
     while ((i=is.read()) != -1)
       if (i==0xFF && is.read()==0xE1) {
         result = checkNamespace(is);
         if (result!=null) break;
       }
     return result;
   }

   public static final String namespace = 
     "http://ns.adobe.com/xap/1.0/";
   
   private String checkNamespace(FileInputStream is) 
      throws IOException {
     String s = getSegment(is);
     return (s.startsWith(namespace)) ? s.substring(29) : null;
   }
   
   private String getSegment(InputStream is) 
      throws IOException {
     DataInputStream dis = new DataInputStream(is);
     byte[] buffer = new byte[dis.readShort()-2];
     dis.read(buffer);
     return new String(buffer);
   }
}

Resource Consumption Management API

Resource Consumption Management API JSR

This sounds like JINI or JMX to me...the JCP is becomming an echo chamber.

Simple template replacer

Often I find I need a simple template engine to replace named regions.

public abstract class Replace {

   public static void main(String[] args) {
      Replace r = new Replace() {
         public String lookup(String key) {
            return "*newvalue*";
         }
      };
      String s =
    "Welcome ${name}, you have just won a new ${prize}.";
      System.out.println(r.replace(s));
   }

   public String replace(String s) {
      return replace(s, 0, new StringBuilder()).toString();
   }

   private StringBuilder replace(String s, int offset, StringBuilder b) {
      int startKey = s.indexOf("${", offset);
      if (startKey == -1)
         return b.append(s.substring(offset));
      else {
         b.append(s.substring(offset, startKey));
         int keyEnd = s.indexOf('}', startKey);
         String key = s.substring(startKey + 2, keyEnd);
         b.append(lookup(key));
         return replace(s, keyEnd + 1, b);
      }
   }

   abstract public String lookup(String key);
}