Using Spring 3 autowire in a standalone Java application
Spring is moving away from XML files and uses annotations heavily. The following example is a simple standalone Spring application which uses annotation instead of XML files.
package com.zetcode.bean;
import org.springframework.stereotype.Component;
@Component
public class Message {
private String message = "Hello there!";
public void setMessage(String message){
this.message = message;
}
public String getMessage(){
return message;
}
}
This is a simple bean. It is decorated with the @Component
annotation for auto-detection by Spring container.
package com.zetcode.main;
import com.zetcode.bean.Message;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
@ComponentScan(basePackages = "com.zetcode")
public class Application {
public static void main(String[] args) {
ApplicationContext context
= new AnnotationConfigApplicationContext(Application.class);
Application p = context.getBean(Application.class);
p.start();
}
@Autowired
private Message message;
private void start() {
System.out.println("Message: " + message.getMessage());
}
}
This is the main Application
class. The @ComponentScan
annotation searches for components. The @Autowired
annotation injects the bean into the message
variable. The AnnotationConfigApplicationContext
is used to create the Spring application context.
My Standalone Spring tutorial shows how to create a standalone Spring application with both XML and annotations.
Spring works in standalone application. You are using the wrong way to create a spring bean. The correct way to do it like this:
@Component
public class Main {
public static void main(String[] args) {
ApplicationContext context =
new ClassPathXmlApplicationContext("META-INF/config.xml");
Main p = context.getBean(Main.class);
p.start(args);
}
@Autowired
private MyBean myBean;
private void start(String[] args) {
System.out.println("my beans method: " + myBean.getStr());
}
}
@Service
public class MyBean {
public String getStr() {
return "string";
}
}
In the first case (the one in the question), you are creating the object by yourself, rather than getting it from the Spring context. So Spring does not get a chance to Autowire
the dependencies (which causes the NullPointerException
).
In the second case (the one in this answer), you get the bean from the Spring context and hence it is Spring managed and Spring takes care of autowiring
.