Lookup method resolution failed; nested exception is java.lang.illegalstateexception: failed to introspect class

When you encounter the error message “lookup method resolution failed; nested exception is
java.lang.illegalstateexception: failed to introspect class,” it means that the Spring framework
encountered an issue while trying to resolve a lookup method and introspect the corresponding class.
This error typically occurs when there are problems with the configuration or setup of your Spring
application.

Here is an example to help illustrate this issue:

        
public class Foo {
    private Bar bar;
    
    public void setBar(Bar bar) {
        this.bar = bar;
    }
    
    public Bar getBar() {
        return bar;
    }
}
        
    

        
public interface Bar {
    void doSomething();
}
        
    

        
public class BarImpl implements Bar {
    public void doSomething() {
        System.out.println("Doing something...");
    }
}
        
    

        
public class MyApp {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        Foo foo = (Foo) context.getBean("foo");
        Bar bar = foo.getBar();
        bar.doSomething();
    }
}
        
    

Let’s assume that the “applicationContext.xml” file contains the following bean definitions:

        
<bean id="foo" class="com.example.Foo">
    <lookup-method name="getBar" bean="bar"/>
</bean>

<bean id="bar" class="com.example.BarImpl"/>
        
    

In this example, we are using the lookup method injection technique to replace the normal bean
initialization for the “bar” dependency within the “foo” bean. However, if the Spring framework
fails to introspect the “BarImpl” class properly, it will throw a
“lookup method resolution failed; nested exception is java.lang.illegalstateexception: failed to introspect class”
error.

To resolve this issue, you should check the following:

• Make sure that the class being used for the lookup method (“BarImpl” in this case) is correctly
  defined and accessible within your project.
• Check if there are any mismatched or missing dependencies in your bean definitions or
  configuration files (e.g., “applicationContext.xml”).
• Ensure that the necessary dependencies and libraries are correctly included in your project’s
  classpath.
• Verify if the lookup method is correctly defined in your bean configuration and that the bean
  being looked up has a proper implementation class.
• If you are using an older version of Spring, consider upgrading to a newer version as it may
  include bug fixes or improvements related to lookup method injection.