Configurable OSGI Service using Felix SCR Annotations

gistfile1.txt

package com.sample.osgi;

import java.util.Map;

import org.apache.felix.scr.annotations.Activate;
import org.apache.felix.scr.annotations.Component;
import org.apache.felix.scr.annotations.Modified;
import org.apache.felix.scr.annotations.Property;

@Component(label = "Service Label", description = "Service Description", metatype = true, immediate = true)
public class ConfigurableService {
  
	@Property(value="default value", label = "Sample Parameter", description = "Example of a component parameter")	
	private static final String SAMPLE_PARAM_NAME = "param.one"; 

	@Activate
	protected void activate(final Map<String, Object> props) {
		this.update(props);
	}

	@Modified
	protected void update(final Map<String, Object> props) {		
		System.out.println(props.get(SAMPLE_PARAM_NAME));
	}

}

why do we use @Property??

and @activate

when will update method called?