Archive for the ‘Java’ Category

Download/Attach source-code/java-docs with maven dependencies

I am using Maven in my projects from last couple of years, and the automatically downloading the Jars from repository feature of maven is really helpful for developers. But If you are using Eclipse and want to read/analyse Source Code or Java Doc of API then you need the jar file that contains the Source code and java doc of the API, but unfortunately maven does not download and attach the source code and java doc of the APIs automatically.

Maven provides some different ways to download and attach that source code and Java Doc:

  • Using maven eclipse plugin
  • Using maven dependency plugin

Note: The sources and javadocs of the libraries must exist in the repository so that the plugin can download it and attach it.

1. Maven eclipse plugin:

Maven dependencies that are deployed with the source and javadocs can be downloaded and attached to the Eclipse library by using maven-eclipse-plugin. It can be done by:

  • passing command-line argument to the maven-eclipse-plugin, or
  • by declaring in the pom.xml

1.1 passing command-line argument to maven-eclipse-plugin:

This example shows that how to do this by passing command line argument to the maven-eclipse-plugin:

mvn eclipse:eclipse -DdownloadSources=true -DdownloadJavadocs=true

1.2 declaring in the pom.xml

This sample pom shows that how to declare downloadSources and downloadJavadocs configuration in pom.xml

<project>

<build>

<plugins>

<plugin>

<groupId>org.apache.maven.plugins</groupId>

<artifactId>maven-eclipse-plugin</artifactId>

<configuration>

<downloadSources>true</downloadSources>

<downloadJavadocs>true</downloadJavadocs>

</configuration>

</plugin>

</plgins>

</build>

</project>

2. Maven dependency plugin:

maven-dependency-plugin provides a goal named sources that resolves the project source dependencies from the repository.

usage:

mvn dependency:sources

This is useful when you want the source attachments downloaded to your local repository and you don’t want to use the eclipse plugin to do this since the eclipse plugin creates/overwrites the eclipse files.

Accessing Private Members of a Java class using Reflection

Somebody asked me that, “Can we access private members of a Java Class?

Then I got the idea to write this article, because Java Reflection API provides a feature to access private members of a Java Class.

java.lang.reflect package provides the AccessibleObject class that is parent class of Constructor, Method, and Field class. Using the AccessibleObject class, we can change the access control flags of a Reflection Object (Reflection is a feature in the Java programming language. It allows an executing Java program to examine or “introspect” upon itself, and manipulate internal properties of the program. For example, it’s possible for a Java class to obtain the names of all its members and display them.), and the setAccessible(boolean) method is used to change the access control flag of an Object.

package test;

/**

* @author girish.gaurav

*

*/

class DemoClass {

private int privateInt = 10;

private String privateString = “temp String”;

private long privateLong = 1234567890L;

private void resetFields(int i, String s, long l) {

privateInt = i;

privateString = s;

privateLong = l;

}

public void display() {

System.out.println(“Int = ” + privateInt);

System.out.println(“String = ” + privateString);

System.out.println(“Long = ” + privateLong);

}

}

class Test {

/**

* @param args

*/

public static void main(String[] args) {

DemoClass obj = new DemoClass();

obj.display();

Class clazz = obj.getClass();

try {

Field intField = clazz.getDeclaredField(“privateInt”);

Field strField = clazz.getDeclaredField(“privateString”);

Field longField = clazz.getDeclaredField(“privateLong”);

intField.setAccessible(true);

strField.setAccessible(true);

longField.setAccessible(true);

System.out.println(“value of privateInt field is : ” + intField.get(obj));

System.out.println(“value of privateString field is : ” + strField.get(obj));

System.out.println(“value of privateLong field is : ” + longField.get(obj));

intField.set(obj, 100);

strField.set(obj, “this is new string”);

longField.set(obj, 55555L);

obj.display();

} catch (SecurityException e) {

e.printStackTrace();

} catch (NoSuchFieldException e) {

e.printStackTrace();

} catch (IllegalArgumentException e) {

e.printStackTrace();

} catch (IllegalAccessException e) {

e.printStackTrace();

}

try {

Method method = clazz.getDeclaredMethod(“resetFields”, int.class, String.class, long.class);

method.setAccessible(true);

method.invoke(obj, 25, “new String from resetField method.”, 987654321L);

obj.display();

} catch (SecurityException e) {

e.printStackTrace();

} catch (NoSuchMethodException e) {

e.printStackTrace();

} catch (IllegalArgumentException e) {

e.printStackTrace();

} catch (IllegalAccessException e) {

e.printStackTrace();

} catch (InvocationTargetException e) {

e.printStackTrace();

}

}

}

Introduction to Nonblocking Sockets

Here, I am going to explain what non-blocking sockets are, how it works, and where it can be useful, everything in details.

Preface:

Non-blocking sockets are introduced in Java 2 Standard Edition 1.4. It allows network communication between applications without blocking processes using the sockets.

A nonblocking socket allows input/output operation on a channel without blocking the processes using it. I’m talking about asynchronous high-performance read/write operations that, as you will see, turn upside-down the techniques for designing and developing socked-based applications.

Java developers who already working or worked on sockets might ask that, “Why we use a new technology to work on Sockets while we already have an old one that working fine? Or whats wrong with the traditional (Java 1.3.x) socket programming? and What are the advantage with new non-blocking socket API?”

Suppose we are going to implement a server application that accepts huge number of client connections and as well as, we want the server that can be able to process multiple requests simultaneously. If we use older socket programming to achieve this requirement we have two ways:

1. Implement a multithread server that manually handles a thread for each connection.
2. By using an external third-party module.

Both the given solution can work fine, but if we adopt first solution then the whole thread-management solution will be developed by the programmer and all the concurrency related issues will also be considered by the programmer. and the second solution may cost money. And by using Non-blocking sockets, you can implement a nonblocking server without directly managing threads or resorting to external modules.