Working with the JRDataSource Interface

Working with the JRDataSource Interface

All data adapters implement the JRDataSource interface. Some data adapters, such the JDBC data connections, do this indirectly using a connection and a query; other data adapters, such as adapters for CSV files, XML documents, and collections of JavaBeans, do this directly. This section is useful if you want to understand more about the direct data adapters, or if you are interested in creating a custom data adapter.

Understanding the JRDataSource Interface

Data supplied by a JRDataSource is ideally organized into records as in a table. Every JRDataSource must implement the following two methods:

public boolean next() – Returns true if the cursor is positioned correctly in the subsequent record, false if no more records are available.

public Object getFieldValue(JRField jrField) – Moves a virtual cursor to the next record

Every time JasperReports executes the public boolean next() method, all the fields declared in the report are filled and all the expressions (starting from those associated with the variables) are calculated again. Subsequently, JasperReports determines whether to print the header of a new group, to go to a new page, and so on. When next returns false, the report is ended by printing all final bands (Group Footer, Column Footer, Last Page Footer, and Summary). The method can be called as many times as there are records present (or represented) from the data source instance.

The method public Object getFieldValue(JRField jrField) is called by JasperReports after a call to next results in a true value. In particular, it's executed for every single field declared in the report. In the call, a JRField object is passed as a parameter. It's used to specify the name, the description and the type of the field from which to obtain the value (all this information, depending on the specific data source implementation, can be combined to extract the field value).

The type of the value returned by the public Object getFieldValue(JRField jrField) method has to be adequate for that declared in the JRField parameter, except when a null is returned. If the type of the field was declared as java.lang.Object, the method can return an arbitrary type. In this case, if required, a cast can be used in the expressions. A cast is a way to dynamically indicate the type on an object, the syntax of a cast is:


in example:


Usually a cast is required when you need to call a method on the object that belongs to a particular class.

Implementing a New JRDataSource

If the JRDataSource supplied with JasperReports doesn't meet your requirements, you can write a new JRDataSource. This is not a complex operation. In fact, all you have to do is create a class that implements the JRDataSource interface that exposes two simple methods: next and getFieldValue.

The JRDataSource interface

package net.sf.jasperreports.engine;
public interface JRDataSource
			public boolean next() throws JRException;
			public Object getFieldValue(JRField jrField) throws JRException;

The next method is used to set the current record into the data source. It has to return true if a new record to elaborate exists; otherwise it returns false.

If the next method has been called positively, the getFieldValue method has to return the value of the requested field or null. Specifically, the requested field name is contained in the JRField object passed as a parameter. Also, JRField is an interface through which you can get information associated with a field—the name, description, and Java type that represents it.

Now try writing your personalized data source. You have to write a data source that explores the directory of a file system and returns the found objects (files or directories). The fields you create to manage your data source are the same as the file name, which should be named FILENAME; a flag that indicates whether the object is a file or a directory, which should be named IS_DIRECTORY; and the file size, if available, which should be named SIZE.

You data source should have two constructors: the first receives the directory to scan as a parameter; the second has no parameters and uses the current directory to scan.

Once instantiated, the data source looks for the files and the directories present in the way you indicate and fills the array files.

The next method increases the index variable you use to keep track of the position reached in the array files, and returns true until you reach the end of the array.

Sample personalized data source

import net.sf.jasperreports.engine.*;
public class JRFileSystemDataSource implements JRDataSource
File[] files = null;
int    index = -1;
public JRFileSystemDataSource(String path)
File dir = new File(path);
if (dir.exists() && dir.isDirectory())
files = dir.listFiles();
public JRFileSystemDataSource()
public boolean next() throws JRException
if (files != null && index < files.length)
return true;
return false;
				public Object  getFieldValue(JRField jrField) throws JRException
				File f = files[index];
				if (f == null) return null;
				if (jrField.getName().equals("FILENAME"))
				return f.getName();
				else if (jrField.getName().equals("IS_DIRECTORY"))
				return new Boolean(f.isDirectory());
				else if (jrField.getName().equals("SIZE"))
				return new Long(f.length());
				// Field not found...
				return null;

The getFieldValue method returns the requested file information. Your implementation doesn't use the information regarding the return type expected by the caller of the method. It assumes the name has to be returned as a string. The flag IS_DIRECTORY as a Boolean object, and the file size as a Long object.

The next section shows how to use your personalized data source in Jaspersoft Studio and test it.

Using a Custom JasperReports Data Source with Jaspersoft Studio

Jaspersoft Studio provides a special connection for your personalized data sources. It's useful for employing whatever JRDataSource you want to use through some kind of factory class that provides an instance of that JRDataSource implementation. The factory is just a simple Java class useful for testing your data source and filling a report in Jaspersoft Studio. The idea is the same as what you have seen for the collection of JavaBeans data adapter — you need to write a Java class that creates the data source through a static method and returns it. For example, if you want to test the JRFileSystemDataSource in the previous section, you need to create a simple class like that shown in this code sample:

Class for testing a custom data source

import net.sf.jasperreports.engine.*;
public class FileSystemDataSourceFactory {
	public static JRDataSource createDatasource()
return new JRFileSystemDataSource("/");

This class, and in particular the static method that's called, executes all the necessary code for instancing the data source correctly. In this case, you create a new JRFileSystemDataSource object by specifying a way to scan the directory root ("/").

Now that you have defined the way to obtain the JRDataSource you prepared and the data source is ready to be used, you can create the connection through which it can be used.

Create a new connection as you normally would (see Working with Database JDBC Connections), then select Custom implementation of JRDataSource from the list and specify a data source name like TestFileSystemDataSource (or whatever name you want), as shown below.

Configuring a Custom Data Adapter

Next, specify the class and method to obtain an instance of your JRFileSystemDataSource, that is, TestFileSystemDataSource and test.

There is no automatic method to find the fields managed by a custom data source.

In this case, you know that the JRFileSystemDataSource provides three fields: FILENAME (String), IS_DIRECTORY (Boolean), and SIZE (Long). After you have created these fields, insert them in the report’s Detail band.

Divide the report into two columns and in the Column Header band, insert Filename and Size tags. Then add two images, one representing a document and the other an open folder. In the Print when expression setting of the Image element placed in the foreground, insert the expression $F{IS_DIRECTORY}, or use as your image expression a condition like the following:

($F{IS_DIRECTORY}) ? “folder.png” : “file.png”

In this example, the class that instantiated the JRFileSystemDataSource was very simple. But you can use more complex classes, such as one that obtains the data source by calling an Enterprise JavaBean or by calling a web service.