Saturday, February 23, 2013

JSP, Servlet, MVC in a nutshell

In a nutshell, this is how you can use JSP and Servlets with the MVC pattern.  

  • Model: A POJO (Email.java) 
  • View: A JSP (email-list.jsp)  
  • Controller: A Servlet (EmailListServlet.java)

Email.java (The Model)


public class Email {
    private String from;
    private String subject;
    private String message;
 
    public Email(String from, String subject, String message) {
 this.from = from;
 this.subject = subject;
 this.message = message;
    }
    
    // Getters and Setters...
    
}

email-list.jsp (The View)


<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Email List</title>
</head>

<body>
    <table>
        <tr><th>From</th><th>Subject</th><th>Message</th></tr>
 
        <c:forEach var="email" items="${emails}">
     <tr>
         <td>${email.from}</td>
  <td>${email.subject}</td>
  <td>${email.message}</td>
     </tr>
 </c:forEach>
    </table>
</body>
</html>

EmailListServlet.java (The Controller)


public class EmailListServlet extends HttpServlet {
 
 private static final long serialVersionUID = 1L;
 
 
 @Override
 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
  
  List<Email> emails = yourMethodThatReturnsEmailList();
  req.setAttribute("emails", emails);
  getServletContext().getRequestDispatcher("/email-list.jsp").forward(req, resp);
 }

}

The servlet mappings in web.xml


<servlet>
  <servlet-name>EmailList</servlet-name>
  <servlet-class>com.test.servlet.EmailListServlet</servlet-class>
</servlet>

<servlet-mapping>
  <servlet-name>EmailList</servlet-name>
  <url-pattern>/email</url-pattern>
</servlet-mapping>

Then just access the servlet i.e www.mydomain.com/email
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Monday, June 4, 2012

Do not track your own views on Blogger

If you do not want to track your own page views on your blog, you'll need to accept a cookie that will prevent it.

After you log in, choose a blog, then from the menu that appears on the left click on “Stats”.   On the right hand side of the page you should see a list with page view counts “Pageviews today”, “Pageviews yesterday”, below the last item on the list (“Pageviews all time history”) you’ll see a link “Don’t track your own pageviews”, click on that link and from the dialog that appears select “Don’t track my page views” and then “Save”.

Now when you visit your own blog your views won’t be tracked in the stats.
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Saturday, March 10, 2012

SQLite, JPA (Hibernate implementation) and Pagination

I was having issues with a Wicket AjaxFallbackDefaultDataTable, where I couldn't get the pagination working properly with an SQLite database.

The relevant code I was using for pagination was this:

 String query = "SELECT p FROM Person p ORDER BY p." 
          + getSort().getProperty() 
          + " " 
          + ascDesc;
TypedQuery personQuery= em.createQuery(query, Person.class);
personQuery.setFirstResult(first);  // first is passed in by the datatable
personQuery.setMaxResults(count);   // count is passed in by the datatable

It turned out that the problem was with the SQLite dialect I was using.

Using Log4JDBC I was able to view the values in the LIMIT and OFFSET clauses in the SQL statements that JPA was creating and executing.  So instead of seeing this:

LIMIT ? OFFSET ?

I could now see this:

LIMIT 10 OFFSET 0

When setting the firstResult to 10 personQuery.setFirstResult(10) and setMaxResults to 6 personQuery.setMaxResults(6) I noticed that the SQL statement was reversing the arguments to the LIMIT and OFFSET clauses, so instead of getting what I was expecting LIMIT 6 OFFSET 10 I was getting LIMIT 10 OFFSET 6. 

The solution was to override the bindLimitParametersInReverseOrder() method and set it to return true and that allowed my pagination to start working as expected.
Note: You can't change the getLimitString method to return LIMIT and OFFSET in reverse order. That causes a syntax error.
Here's the slightly modified SQLite dialect (I found the original on the net) that will work with pagination.


import java.sql.Types;

import org.hibernate.dialect.Dialect;
import org.hibernate.dialect.function.StandardSQLFunction;
import org.hibernate.dialect.function.SQLFunctionTemplate;
import org.hibernate.dialect.function.VarArgsSQLFunction;
import org.hibernate.Hibernate;

public class SQLiteDialect extends Dialect {
  public SQLiteDialect() {
    super();
    registerColumnType(Types.BIT, "integer");
    registerColumnType(Types.TINYINT, "tinyint");
    registerColumnType(Types.SMALLINT, "smallint");
    registerColumnType(Types.INTEGER, "integer");
    registerColumnType(Types.BIGINT, "bigint");
    registerColumnType(Types.FLOAT, "float");
    registerColumnType(Types.REAL, "real");
    registerColumnType(Types.DOUBLE, "double");
    registerColumnType(Types.NUMERIC, "numeric");
    registerColumnType(Types.DECIMAL, "decimal");
    registerColumnType(Types.CHAR, "char");
    registerColumnType(Types.VARCHAR, "varchar");
    registerColumnType(Types.LONGVARCHAR, "longvarchar");
    registerColumnType(Types.DATE, "date");
    registerColumnType(Types.TIME, "time");
    registerColumnType(Types.TIMESTAMP, "timestamp");
    registerColumnType(Types.BINARY, "blob");
    registerColumnType(Types.VARBINARY, "blob");
    registerColumnType(Types.LONGVARBINARY, "blob");
    // registerColumnType(Types.NULL, "null");
    registerColumnType(Types.BLOB, "blob");
    registerColumnType(Types.CLOB, "clob");
    registerColumnType(Types.BOOLEAN, "integer");

    registerFunction("concat", new VarArgsSQLFunction(Hibernate.STRING, "",
        "||", ""));
    registerFunction("mod", new SQLFunctionTemplate(Hibernate.INTEGER,
        "?1 % ?2"));
    registerFunction("substr", new StandardSQLFunction("substr",
        Hibernate.STRING));
    registerFunction("substring", new StandardSQLFunction("substr",
        Hibernate.STRING));
  }

  public boolean supportsIdentityColumns() {
    return true;
  }

  public boolean hasDataTypeInIdentityColumn() {
    return false; // As specify in NHibernate dialect
  }

  public String getIdentityColumnString() {
    // return "integer primary key autoincrement";
    return "integer";
  }

  public String getIdentitySelectString() {
    return "select last_insert_rowid()";
  }

  public boolean supportsLimit() {
    return true;
  }

  public String getLimitString(String query, boolean hasOffset) {
    return new StringBuffer(query.length() + 20).append(query).append(
        hasOffset ? " limit ? offset ?" : " limit ?").toString();
  }
  
  public boolean bindLimitParametersInReverseOrder() {
    return true;
  }

  public boolean supportsTemporaryTables() {
    return true;
  }

  public String getCreateTemporaryTableString() {
    return "create temporary table if not exists";
  }

  public boolean dropTemporaryTableAfterUse() {
    return false;
  }

  public boolean supportsCurrentTimestampSelection() {
    return true;
  }

  public boolean isCurrentTimestampSelectStringCallable() {
    return false;
  }

  public String getCurrentTimestampSelectString() {
    return "select current_timestamp";
  }

  public boolean supportsUnionAll() {
    return true;
  }

  public boolean hasAlterTable() {
    return false; // As specify in NHibernate dialect
  }

  public boolean dropConstraints() {
    return false;
  }

  public String getAddColumnString() {
    return "add column";
  }

  public String getForUpdateString() {
    return "";
  }

  public boolean supportsOuterJoinForUpdate() {
    return false;
  }

  public String getDropForeignKeyString() {
    throw new UnsupportedOperationException(
        "No drop foreign key syntax supported by SQLiteDialect");
  }

  public String getAddForeignKeyConstraintString(String constraintName,
      String[] foreignKey, String referencedTable, String[] primaryKey,
      boolean referencesPrimaryKey) {
    throw new UnsupportedOperationException(
        "No add foreign key syntax supported by SQLiteDialect");
  }

  public String getAddPrimaryKeyConstraintString(String constraintName) {
    throw new UnsupportedOperationException(
        "No add primary key syntax supported by SQLiteDialect");
  }

  public boolean supportsIfExistsBeforeTableName() {
    return true;
  }

  public boolean supportsCascadeDelete() {
    return false;
  }
}
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