Learn Java by Examples
This example show you how to get operating system active user’s login name. We can obtain the username of current user by reading system properties using the user.name key.
package org.kodejava.lang;
public class GettingUserName {
public static void main(String[] args) {
String username = System.getProperty("user.name");
System.out.println("username = " + username);
}
}
This example show you how to obtain web application context path in JSP using Expression Language (EL) feature of JSP. To get the context path we can utilize the pageContext, it is an implicit object that available on every JSP pages. From this object you can get access to various object such as:
servletContextsessionrequestresponseTo get the context path value you will need to read it from the request.contextPath object. This contextPath can be useful for constructing a path to you web resources such as CSS, JavaScript and images. Libraries that you’ll need to enable the JSP Expression Language (EL) in your JSP Pages, which usually already included in a Servlet container such as Apache Tomcat.
Here is our context-path.jsp file.
<%@ page contentType="text/html;charset=UTF-8" %>
<!DOCTYPE html>
<html lang="en">
<head>
<title>JSP - Context Path</title>
</head>
<body>
Web Application Context Path = ${pageContext.request.contextPath}
</body>
</html>
Maven dependencies
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>4.0.1</version>
</dependency>
The context path always comes first in a request URI. The path starts with a “/” character but does not end with a “/” character. When I have a web application with the URL like http://localhost:8080/myapp` then/myapp` is the context path.
For servlets in the default (root) context, this method returns "" (empty string).
package org.kodejava.servlet;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import java.io.IOException;
import java.io.PrintWriter;
@WebServlet(urlPatterns = "/context-path")
public class ContextPathDemo extends HttpServlet {
protected void doGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
// HttpServletRequest.getContextPath() returns the portion
// of the request URI that indicates the context of the
// request.
String contextPath = req.getContextPath();
PrintWriter pw = res.getWriter();
pw.print("Context Path: " + contextPath);
}
}
You’ll get the following information in your browser:
Context Path: /webapp
Maven dependencies
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>4.0.1</version>
</dependency>
This example demonstrates how to copy file using the Java IO library. Here we will use the java.io.FileInputStream and it’s tandem the java.io.FileOutputStream class.
package org.kodejava.io;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
public class FileCopyDemo {
public static void main(String[] args) {
// Create an instance of source and destination files
File source = new File("source.pdf");
File destination = new File("target.pdf");
try (FileInputStream fis = new FileInputStream(source);
FileOutputStream fos = new FileOutputStream(destination)) {
// Define the size of our buffer for buffering file data
byte[] buffer = new byte[4096];
int read;
while ((read = fis.read(buffer)) != -1) {
fos.write(buffer, 0, read);
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
The System.clearProperty(String key) method enables you to remove a system property. The key must not be an empty string or a null value because it will cause the method to throw an IllegalArgumentException or a NullPointerException.
It will also check if a SecurityManager exists and if you don’t have a write permission to the system property a SecurityException is going to be thrown.
package org.kodejava.lang;
public class ClearProperty {
public static void main(String[] args) {
String key = "user.dir";
System.out.println(key + " = " + System.getProperty(key));
// The System.clearProperty() method available since Java 1.5
System.clearProperty(key);
System.out.println(key + " = " + System.getProperty(key));
}
}
The code snippet above give us the following output:
user.dir = F:\Wayan\Kodejava\kodejava-example
user.dir = null