Archive for the ‘java’ Category

Files and directories in Java: the NIO.2 Approach

In the previous post I told you about the old fashioned way to handle files in java.
In this article we will focus on the new API.
The File class is no longer used. We will use the “Path” class instead.
Files is a utility class to create either files or directories, that takes a path in the constructor:

	Path filePath=Paths.get("/home/laura/demo.txt");
	Files.createFile(filePath);
	
	Path dirPath=Paths.get("/home/laura/newdir");
	Files.createDirectory(dirPath);
	
	Path nestedDirPath=Paths.get("/home/laura/newdir3/newdir4");
	Files.createDirectories(nestedDirPath);

The “createDirectories” method created the whole file system structure (the nested directories).

With NIO.2 copying or deleting a file or directory have become a matter of one line of code, thanks to the very straightforward methods “copy”, “delete”, etc.

Many things can be achieved with the utility classes Files and Paths.

Java IO: Handling characters files efficiently

Let´s take a look a the classic IO file handling in java.
First let´s start with the “File” class.
The File class is not used to read or write data. It´s used to create or delete files and directories, for searching and working with paths.
So it´s not used for the files or directories content.

The File class constructor does not create anything on the hard drive. As you can see in the following snippet, you need to use the createNewFile method for it:

		File file = new File("/home/laura/demo.txt");
		System.out.println("Does the file exists?"+file.exists());
		boolean created = file.createNewFile();
		System.out.println("File created?"+created);
		System.out.println("Does the file exists?"+file.exists());

The createNewFile method returns “false” if the file already exists.

If the goal is handling text files, you don´t need to use any Stream class. All you need is a writer one. The basic option is to use the naked “FileReader” and “FileWriter” classes:

		FileWriter fileWriter= new FileWriter(file);
		fileWriter.write("hello world. this is a text file.");
		fileWriter.flush();
		fileWriter.close();
		
		FileReader fileReader= new FileReader(file);
		char[] in = new char[100];		
		int size = fileReader.read(in);
	
		System.out.println("File size:"+size);
		
		for(char c: in){
			System.out.print(c);
		}
	
		fileReader.close();

The FileReader method “read” delivers the amount of character read.

Notice that after writing the text into the file, you need to call the method “flush”. It´s necessary to call it to make sure that the whole data flows into the file before closing the writer.

So far so good. But this simple way shown above is not the most elegant solution out there, because we have been using an array (that has a fixed size)!

A much better approach is wrapping it up using the classes BufferedReader and BufferedWriter:

		FileWriter fileWriter= new FileWriter(file);
		BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
		
		bufferedWriter.write("hello world. this is a text file.");
		bufferedWriter.newLine();
		bufferedWriter.write("You are using the buffered reader now!");			
		bufferedWriter.close();
		
		FileReader fileReader= new FileReader(file);
		BufferedReader bufferedReader = new BufferedReader(fileReader);
		
		String data;			
		while((data=bufferedReader.readLine())!=null){
			System.out.println(data);		
		}
		
		bufferedReader.close();

Notice that buffered reader also has a method called “readLine”, that you wouldn´t get with a simple File reader.
Once you close a writer it cannot be reopened again. You get an exception if you use it after closing it!

Debugging and Testing in Java : enabling assertions

For testing and debugging purposes you can enable the assertions evaluation with the VM parameter “-ea” ( or “-enableassertions” if you prefer the whole thing).

The assertions remain in the code and are just ignored at runtime if you don´t enable them. You can think of them as an aid in case of need.

The following method throws an exception if the parameter given is not a String containing “Laura”:


         private void testFirstName(String firstName){
                 assert(firstName.equals("Laura"));

               System.out.println("The given first name is Laura");
         }

A string message can be displayed in the stack trace, by adding a second expression to the assertion, like :

	private void testFirstName(String firstName){
		assert(firstName.equals("Laura")) : "the name "+ firstName+ " is not Laura!";
		
		System.out.println("The given first name is Laura");
	}

The output in this second case would be something like:

Exception in thread "main" java.lang.AssertionError: the name Anna is not Laura!
at com.demo.AssertionsDemo.testFirstName(AssertionsDemo.java:15)
at com.demo.AssertionsDemo.main(AssertionsDemo.java:8)

Since Java 1.4 “assert” has become a keyword.

Assertions can be disabled with the VM option “-da” or “-disableassertions”. Although the assertions are disabled by default, the manual disabling might make sense if you don´t want to enable the assertions for all the classes.

For example if you want to run a java jar file (for example “demo.jar”), but disabling the assertions for one of its classes (ex. “com.demo.Demo.java”) you need to use the following parameters:

> java -jar -ea -da:com.demo.Demo

To disable them for the whole “com.demo” package (including subpackages):

> java -jar -ea -da:com.demo...

So entering VM parameters like “-ea:” or “-ea:” allow you to select which assertions to evaluate.

Java: the Comparable and Comparator interfaces

The Comparable interface allows you to sort by a specific object variable that you choose for your needs.
Take a look at the following object:

public class Book implements Comparable<Book> {
	
	private String title;
	private String author;
	private double price;
	
	public Book(String title, String author, double price) {
		super();
		this.title = title;
		this.author = author;
		this.price = price;
	}

	public String getAuthor() {
		return author;
	}

	public void setAuthor(String author) {
		this.author = author;
	}


	public double getPrice() {
		return price;
	}

	public void setPrice(double price) {
		this.price = price;
	}

	public String getTitle() {
		return title;
	}

	public void setTitle(String title) {
		this.title = title;
	}


	@Override
	public int compareTo(Book that) {
		return Double.valueOf(this.price).compareTo(Double.valueOf(that.price));
	}

	public String toString() {
		return " \n " + title + " \t " + author + " \t " + price;
		}
		
}

Notice the way the compareTo method is overriden. It´s a double primitive variable, but you need the wrapper class to compare it. The following wouldn´t work:

	@Override
	public int compareTo(Book that) {
		return this.price.compareTo(that.price);
		return 0;
	}

Implementing the Comparable interface affects the sorting criteria. For example:

import java.util.Arrays;

public class ComparableTest{
	
	public static void main(String[] args) {
		
		Book[] books= {
		new Book("The capital","Marx,Karl",10.5),
		new Book("Interpretation of dreams","Freud, S.",12.5),
		new Book("Killed by Death","Kilmister, Lemmy",8)
		};
		
		System.out.println("Before sorting: \n"+Arrays.toString(books));

	Arrays.sort(books);
	System.out.println("\n After sorting: \n"+Arrays.toString(books));
	
	}
	
}

The output is:

Before sorting:
[
The capital Marx,Karl 10.5,
Interpretation of dreams Freud, S. 12.5,
Killed by Death Kilmister, Lemmy 8.0]


After sorting:
[
Killed by Death Kilmister, Lemmy 8.0,
The capital Marx,Karl 10.5,
Interpretation of dreams Freud, S. 12.5]

 

If you need to do a more complex comparison, you can make your own comparator by extending the Comparator interface (not “Comparable”).
For example you can create your own Comparator:

public class BookComparator implements Comparator<Book> {

	@Override
	public int compare(Book b1, Book b2) {
		
		int comparePrice = Double.valueOf(b1.getPrice()).compareTo(Double.valueOf(b2.getPrice()));
		if (comparePrice!=0) 
			{return comparePrice;
			}
		else return b1.getAuthor().compareTo(b2.getAuthor());
	}

}

It tries to order by price first, but ifthe books have the same price, they will be sorted by the author name too.

If you runt the following main method:

	public static void main(String[] args) {

		ArrayList<Book> booksList = new ArrayList<Book>();
		booksList.add(new Book("The capital", "Marx,Karl", 10.5));
		booksList.add(new Book("Peace and War", "Tostoj. L.", 12.5));
		booksList.add(new Book("Interpretation of dreams", "Freud, S.", 12.5));
		booksList.add(new Book("Killed by Death", "Kilmister, Lemmy", 8));
		
		System.out.println("before sorting:");
		for (Book book : booksList) {
			 System.out.println( book.toString());

		}

		Collections.sort(booksList, new BookComparator());
		System.out.println(""
				+ "\n after sorting:");
		
		for (Book book : booksList) {
			 System.out.println(book.toString());

		}

Then you see the following output:

before sorting:
The capital Marx,Karl 10.5
Peace and War Tostoj. L. 12.5
Interpretation of dreams Freud, S. 12.5
Killed by Death Kilmister, Lemmy 8.0

after sorting:
Killed by Death Kilmister, Lemmy 8.0
The capital Marx,Karl 10.5
Interpretation of dreams Freud, S. 12.5
Peace and War Tostoj. L. 12.5

Notice the order of the last two books.

 

Getting started with Java Threads – Part 2 : join() and sleep()

The thread class provides some methods to define the threads behavior.
Two important methods to consider are:
join(), to make the other instanced threads to wait for it to day. You can provide a timeout as parameter.
sleep();

The sleep() method throws a checked exception called “InterruptedException”,each time the thread receives an interrupt request. You have to handle in the code by wrapping the method in a try-catch block.

The following code will make you understand the join method.
We define a MyThread thread:

public class MyThread implements Runnable {

	public void run() {
		Thread thread= Thread.currentThread();
		System.out.println("Implementing the runnable interface"
				+ " - Thread name: " + thread.getName()
				+ " - priority: " + thread.getPriority()
				+ " - group: " + thread.getThreadGroup().getName());

	}

}

Then we can observe the behaviour running the following main:

		public static void main(String[] args) {

		// extending Thread
		Thread myThread = new Thread(new MyThread());
		Thread myJoiningThread = new Thread(new MyThread());

		myThread.setName("myThread");
		myJoiningThread.setName("myJoiningThread");

		System.out.println("Threads will be started...");

		myJoiningThread.start();

		try {
			// waits for this thread to die
			myJoiningThread.join();
		} catch (InterruptedException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
		}
		myThread.start();

	}

Without the try-catch block in which the join method is invoked the threads would be started asynchronously (the order is not predictable) and in the console you might get either:
Threads will be started...
Implementing the runnable interface - Thread name: myThread - priority: 5 - group: main
Implementing the runnable interface - Thread name: myJoiningThread - priority: 5 - group: main

or:

Threads will be started...
Implementing the runnable interface - Thread name: myJoiningThread - priority: 5 - group: main
Implementing the runnable interface - Thread name: myThread - priority: 5 - group: main

But if you include the try-catch block, the myJoiningThread will always be invoked first.

To understand the sleep method you can create a MySleepingThread:

public class MySleepingThread implements Runnable {

	public void run() {
		Thread thread= Thread.currentThread();
		try {
			Thread.sleep(3000);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		System.out.println("Implementing the runnable interface"
				+ " - Thread name: " + thread.getName()
				+ " - priority: " + thread.getPriority()
				+ " - group: " + thread.getThreadGroup().getName());

	}

}

Then you can run the following main method:

public static void main(String[] args) {

		// extending Thread
		Thread myThread = new Thread(new MyThread());
		Thread mySleepingThread = new Thread(new MySleepingThread());
		myThread.setName("myThread");
		mySleepingThread.setName("mySleepingThread");

		System.out.println("Threads will be started...");

		mySleepingThread.start();
		myThread.start();

	}

You can see in the console that the sleeping thread will be started after 3 seconds.

The sleep method is static. By invoking “Thread.sleep(3000)” you are delaying the creation of the current thread.

If you use the method in the main method, the sleeping thread will be “main”.

Getting started with Java Threads – Part 1

I am learning for the Java OCP 7 and summarizing some relevant infos about Multithreading.
For now I am referring to chapter 13 in the book by G.Ganesh.

You can create threads in 2 ways: extending the Thread class or implementing the Runnable interface
Extending the Thread class is more convenient if you don´t need to extend another class that is not “Thread” (since in java you can only extend one class), because you can use the class methods directly (like “getName()“).
If you implement the Runnable interface (preferred way because of inheritance) you will have to use the static method “currentThread()” of the Thread class to do your operations on the thread (like setting or getting the name).

The two fundamental methods to know are:
run(): like a main method, necessary to enable the execution of a thread.
start(): that creates the thread.

The thread will be terminated when the run method execution is complete. It´s invoked implicitly by the JVM (DON´T invoke it explicitly!).
The main method starts a Thread called “main“. If you create and start your own thread it will have default names like “Thread-0“, “Thread-1“.

If you extend the thread class you need to override the run() method (otherwise it does nothing). If you implement the Runnable interface you will have to implement the run() method (mandatory)

The following code will help you to understand more about these first concepts.

public class MyThread1 extends Thread {

	public void run() {
		System.out.println("Extending the Thread class - Thread name: "
				+ getName() + " - priority: " + getPriority() + " - group: "
				+ getThreadGroup().getName());

	}

}
public class MyThread2 implements Runnable {

	public void run() {
		Thread thread= Thread.currentThread();
		System.out.println("Implementing the runnable interface"
				+ " - Thread name: " + thread.getName()
				+ " - priority: " + thread.getPriority()
				+ " - group: " + thread.getThreadGroup().getName());

	}

}
public class ThreadsDemo {

	public static void main(String[] args) {

		// extending Thread
		Thread myThread1 = new MyThread1();
		// implementing Runnable
		Thread myThread2 = new Thread(new MyThread2());

		System.out.println("Threads defined. Default properties: ");
		//name, priority, group returned by the toString method 
		System.out.println(myThread1);
		System.out.println(myThread2);
		System.out.println("Changing default names...");
		myThread1.setName("Thread_1");
		myThread2.setName("Thread_2");
		System.out.println("Threads will be started...");
		myThread1.start();
		myThread2.start();

	}

}

The output of the main method should be something like:

Threads defined. Default properties:
Thread[Thread-0,5,main]
Thread[Thread-1,5,main]
Threads will be started...
Extending the Thread class - Thread name: Thread-0 - priority: 5 - group: main
Implementing the runnable interface - Thread name: Thread-1 - priority: 5 - group: main

Launching the main program you see that the threads are started asynchronously. Sometimes you see:
Implementing the runnable interface - Thread name: Thread-1
Extending the Thread class - Thread name: Thread-0

Sometimes it´s:
Extending the Thread class - Thread name: Thread-0
Implementing the runnable interface - Thread name: Thread-1

You can quickly proof it in Eclipse launching the program with the shortcut Ctlr+11.

To change this “random” behaviour you can set the priority with the setPriority() method, from 1, the lowest, to 10, the highest. The default is 5. They can be all retrieved by the static members:
– Thread.NORM_PRIORITY ;
– Thread.MAX_PRIORITY ;
– Thread.MIN_PRIORITY .

Threads can be in one of the following states:
– NEW (created)
– RUNNABLE (started)
– TERMINATED
– BLOCKED (waiting to acquire the lock)
– WAITING (waiting for notifications)
– TIMED_WAITING (sleep() invoked and the thread is sleeping or if wait() with timeout)

States are defined with the Thread.State enumeration.
You can get the state with the getState() method.

Wildfly Apache Modcluster : Error MODCLUSTER000042

I was dealing with the Apache modcluster configuration for the Wildfly load balancing and I ran into the following error

[Server:server-four] 16:55:00,093 ERROR [org.jboss.modcluster] (UndertowEventHandlerAdapter - 1) MODCLUSTER000042: Error MEM sending STATUS command to localhost/127.0.0.1:6666, configuration will be reset: MEM: Can't read node

The error occurs when the servers are “idle”, that is where there is nore request from the browser for a time longer than the keep alive timeout setting.

Wildfly Standalone clustering

To set up a cluster of standalone servers you have two possibilites: nodes running on different machine (horizontal scaling) or running on the same machine (vertical scaling).

To make a wildfly standalone cluster with 2 nodes on the same machine you can follow the steps below:

1) copy the standalone folder and rename it to „standalone1“, „standalone2

2) create 2 scripts in the /bin folder, to avoid typing all the parameter in the command line each time.

script1.sh

#!/bin/sh
./standalone.sh -Djboss.server.base.dir=$JBOSS_HOME/standalone1 -Djboss.node.name=server1 -c standalone-full-ha.xml

script2.sh

#!/bin/sh
./standalone.sh -Djboss.server.base.dir=$JBOSS_HOME/standalone2 -Djboss.node.name=server2 -c standalone-full-ha.xml -Djboss.socket.binding.port-offset=100

To bind the public interface to a specific address add the IP as parameter like the following :
-b 192.168.1.1
For the management console it´s something like:
-bmanagement=192.168.1.1

To bind to all available IPs set them to „0.0.0.0“ like:
-b 0.0.0.0 -bmanagement=192.168.1.1

The high availability configuration has default multicast addresses set to 230.0.0.4.

They can also be changed in the socket-binding section in the standalone xml file.

Alternatively you can add it as parameter like : -u=230.0.1.2

The port offset must be considered, if you need to run the cli for the second instance specify the port:
./jboss-cli.sh --controller=localhost:10090 --connect

3) Set the hornetQ clustering username and password.

The cluster nodes must have the same user und password otherwise you will get:

ERROR [org.hornetq.core.server] (default I/O-1) HQ224018: Failed to create session:
HornetQClusterSecurityException[errorType=CLUSTER_SECURITY_EXCE
PTION message=HQ119099: Unable to authenticate cluster user: HORNETQ.CLUSTER.ADMIN.USER]

To fix this add the cluster username and password in the subsystem messaging:2.0 for any standalone xml configuration file under the tag hornet1-server :

 <clustered>true</clustered>
<cluster-user>clusteruser</cluster-user>
<cluster-password>cluster-secret</cluster-password>

Otherwise you can set to false if you don ́t need to cluster the messaging.

4) run the scripts in two different terminals (start with server 1).

5) deploy the app.
The clustering service will be initiated only if a cluster-enabled application is deployed.
In the web.xml of the application you need to add the distributable tag:

<distributable/>

You need to deploy the app in every server instance

You can try it with the app by Arun Gupta called „clustering“, that you can find at https://github.com/arun-gupta/wildfly-samples.
To make it a little simpler you can create a maven simple project and copy the content in the webapp folder (https://github.com/arun-gupta/wildfly-samples/tree/master/clustering/http/src/main/webapp).
Then you can edit the pom.xml like the following:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>de.demo</groupId>
  <artifactId>demo-clustered-app</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>war</packaging>
  <name>Demo_Clustered_App</name>

  <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    </properties>

    <dependencies>
        <dependency>
            <groupId>javax</groupId>
            <artifactId>javaee-api</artifactId>
            <version>7.0</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>2.4</version>
                <configuration>
                    <failOnMissingWebXml>false</failOnMissingWebXml>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Run mvn package. In the target folder you will find the .war file, that you can manually deploy in each management console.
At the following urls you will see that the node server names are different for the two urls but the session data is shared:
http://localhost:8080/demo-clustered-app-0.0.1-SNAPSHOT/index.jsp
http://localhost:8180/demo-clustered-app-0.0.1-SNAPSHOT/index.jsp

Once you get your app deployed on both instances in the logging of the server one you will see something like:

INFO [org.infinispan.remoting.transport.jgroups.JGroupsTransport] (Incoming-1,shared=udp) ISPN000094: Received new cluster view: [server1/web|1] (2) [server1/web, server2/web]

Defining a port offset is not the only way to set up a cluster on the same machine.
You can also define multiple IP addresses on the same machine (multihoming).

MySQL CHECK constraint alternative: triggers!

Yesterday I discovered the powerful hibernate check constraint (@org.hibernate.annotations.Check) , which can be directly added in your entity class like this:

@Entity
@XmlRootElement
@Check(constraints = "age IS NOT NULL")
public class Person{

String firstname;
String lastName;
Integer age;

// valid code
}

Unfortunately, as you can read in the official MySQL docs, “the CHECK clause is parsed but ignored by all storage engines”.
Yesterday I found out that MySql 5.X doesn´t support the SQL CHECK constraint.
It means that if you are using JPA und HIBERNATE you can´t take advantage of the Check annotation!

In the project I am working on we could successfully export the schema with the maven command “hibernate4:export”. The check constraint was added in the create table statement. So if your DBMS supports it you get the job done in a very elegant way.

The way out I could find by googling a bit is not so elegant, but it allowed me to achieve the same result. I have just written a trigger, like the (simple) following:

CREATE TRIGGER check_age
BEFORE INSERT ON person
FOR EACH ROW
BEGIN
IF age is NULL
THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Age cannot be null';
END IF;
END;

MySQL has been bought by Oracle. Is this silent ignoring is a strategy to make people migrate to Oracle DBMS? I am afraid it´s so. Corporation games, my friends!

Selenium Web driver tests fail if run all together

If you write unit tests that must be run like if you had to open/refresh a new browser session each time, you can use a method with before annotations:

@Before
public void refreshPage() {
    driver.navigate().refresh();
}

If all tests are individually successful (green) but fail all together, the reason might also been that you need to wait for some resources to be available on the page, so you also need to handle it, setting the timeout like this:

 public WebElement getSaveButton() {
     return findDynamicElementByXPath(By.xpath("//*[@id=\"form:btnSave\"]"), 320);
 }

320 is a long time, but you must make sure that you give enough time to get all that it takes to test.

 

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