Spring Tutorial - Spring Bean Auto Scan

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Spring can auto scan, detect, wire and instantiate Java beans.

The following sections shows the difference between xml configuration vs auto scan auto wire configuration.

Old fashion

Here is a normal Java bean.

package com.java2s.common;

public class Printer 
{
}

Another Java bean with referencing the code above.

package com.java2s.common;

public class PrinterHelper 
{
  Printer printer;
  public void setPrinter(Printer myPrinter) {
    this.printer = myPrinter;
  }

  @Override
  public String toString() {
    return "PrinterHelper [myPrinter=" + printer + "]";
  }
}

XML Bean configuration file Spring-Customer.xml with a normal bean configuration in Spring.

<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
  http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
  <bean id="printerHelper" class="com.java2s.common.PrinterHelper">
    <property name="printer" ref="myPrinter" />
  </bean>
  <bean id="myPrinter" class="com.java2s.common.Printer" />
</beans>

Here is the code to run.

package com.java2s.common;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class App 
{
    public static void main( String[] args )
    {
      ApplicationContext context = 
        new ClassPathXmlApplicationContext(new String[] {"SpringBeans.xml"});
      PrinterHelper cust = (PrinterHelper)context.getBean("printerHelper");
      System.out.println(cust);
    }
}

Auto Java bean Scan and wire

To enable Spring auto component scanning features, add annotation @Component to class.

package com.java2s.common;

import org.springframework.stereotype.Component;
@Component
public class Printer 
{
}

The following Java bean uses not only @Component to indicate this is an auto scan component, it also marks the property as Autowired.

package com.java2s.common;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class PrinterHelper 
{
  @Autowired
  Printer myPrinter;
}

The following xml configuration file finally adds the package to enable the auto wire auto scan context.

<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:context="http://www.springframework.org/schema/context"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
  http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
  http://www.springframework.org/schema/context
  http://www.springframework.org/schema/context/spring-context-2.5.xsd">
  <context:component-scan base-package="com.java2s.common" />
</beans>

Note

By default, Spring will lower case the first character of the component's simple class name as the bean id.

The following code shows how to use the auto-generated id to retrieve the component.

PrinterHelper cust = (PrinterHelper)context.getBean("printerHelper");

We can also name a component when declaring it.

@Component("myService")
public class PrinterHelper {
}

Once we define the name in @Component, we can retrieve it with this name 'myService'.

PrinterHelper cust = (PrinterHelper)context.getBean("myService");

Filter to include

The following code shows how to use Spring filter to scan and register components by regular expressions, even the class is not annotated with @Component.

package com.java2s.common;
public class Printer 
{
}

Helper class

package com.java2s.common;

import org.springframework.beans.factory.annotation.Autowired;

public class PrinterHelper 
{
  @Autowired
  Printer myPrinter;
}

The following xml configuration uses the Spring filter to include classes.

<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:context="http://www.springframework.org/schema/context"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
  http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
  http://www.springframework.org/schema/context
  http://www.springframework.org/schema/context/spring-context-2.5.xsd">

  <context:component-scan base-package="com.java2s" >
    <context:include-filter type="regex" 
                       expression="com.java2s.common.*DAO.*" />
    <context:include-filter type="regex" 
                       expression="com.java2s.common.*Service.*" />
  </context:component-scan>
</beans>

The following code includes any classes under com.java2s.customer.dao package with DAO in class name.

<context:include-filter type="regex" 
                     expression="com.java2s.common.*DAO.*" />

Run it

package com.java2s.common;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.java2s.customer.services.PrinterHelper;
public class App 
{
    public static void main( String[] args )
    {
      ApplicationContext context = 
    new ClassPathXmlApplicationContext(new String[] {"SpringBeans.xml"});
      PrinterHelper cust = (PrinterHelper)context.getBean("printerHelper");
      System.out.println(cust);
    }
}

Filter to exclude

The following code shows how to exclude specified components to stop Spring from auto registering for classes annotated with @Service.

<context:component-scan base-package="com.java2s.common" >
    <context:exclude-filter type="annotation" 
      expression="org.springframework.stereotype.Service" />    
</context:component-scan>

Exclude classes with DAO letters in class name.

<context:component-scan base-package="com.java2s" >
    <context:exclude-filter type="regex" 
      expression="com.java2s.customer.dao.*DAO.*" />    
</context:component-scan>

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