[Spring Boot] Use Component Scan to scan for Bean

Component Scan is important concept when we want to create Bean. 

Currently we know what, for the class, we want to create Bean from it, we need to add @Component.

@Component
@Scope(ConfigurableBeanFactory.SCOPE_SINGLETON)
public class ComponentPersonDAO {

    @Autowired
    ComponentJDBCConnection jdbcConnection;

    public ComponentJDBCConnection getJdbcConnection() {
        return jdbcConnection;
    }

    public void setJdbcConnection(ComponentJDBCConnection jdbcConnection) {
        this.jdbcConnection = jdbcConnection;
    }
}

Another important thing to know that How Spring Boot knows where to find those @Component, this is where @ComponentScan comes into play. by default, it assume search inside the same package. If inside same package cannot find the Bean, then it will report error.

For example, our main function is inside package called:  

com.example.in28minutes

package com.example.in28minutes;

import componentScan.ComponentPersonDAO;
...

@SpringBootApplication
public class In28minutesComponentScanApplication {

    private static Logger LOGGER = LoggerFactory.getLogger(In28minutesComponentScanApplication.class);

    public static void main(String[] args) {
        // Application Context
        ...

    }
}

But where we import ComponentPersonDAO.java from another package: 

componentScan

In this case, the code won't work, it reports that cannot find ComponentPersonDAO bean.

To fix the problem, we can add @ComponentSan("componentScan").

package com.example.in28minutes;

import componentScan.ComponentPersonDAO;

@SpringBootApplication
@ComponentScan("componentScan")
public class In28minutesComponentScanApplication {

    ...

    }
}

 It tells the Spring to scan for component inside "componentScan" package.