Introduce Guice with Example

The motivation of Guice

• Guice alleviates the need for factories and the user of new operator in Java
• Make youself confident with Java's type safe nature.
• Helps you design better API
• Easy tests
• Less boilerplate code
• Works on Java SE, Java EE, Android and GWT

Let me give you a little taste of Guice by an example.

public interface Shorter {
    String shortenUrl(String text);
}

public class UrlShortener implements Shorter {

    public String shortenUrl(String text) {
        // Local or remote URL shorted service request here
        return "short-url";
    }
}

public interface WeiboSender {

    void send(String text);
}

public class WeiboBrowserSender implements WeiboSender {

    @Override
    public void send(String text) {
        // Send weibo.
        System.out.println("Weibo was sent out: " + text);
    }
}

import com.google.inject.Inject;

/**
 * Weibo client for posting, first it will try to shorten url, then send the weibo out.
 * <p/>
 * User: George Sun
 * Date: 7/12/13
 * Time: 7:25 PM
 */
public class WeiboClient {
    private WeiboSender sender;
    private UrlShortener urlShortener;

    // Note the Inject here, this is the key of this example.
    @Inject
    public WeiboClient(WeiboSender sender, UrlShortener urlShortener) {
        this.sender = sender;
        this.urlShortener = urlShortener;
    }

    public void postWeibo(String text) {
        String weiboText = getWeiboText();
        if (weiboText.length() > 140) {
            weiboText = urlShortener.shortenUrl(weiboText);
        }

        if (weiboText.length() <= 140) {
            sender.send(weiboText);
        }
    }

    private String getWeiboText() {
        return "Sample Weibo text here.";
    }
}

import com.google.inject.AbstractModule;

/**
 * Tell Guice what to inject, then Guice will inject instances when needed.
 * <p/>
 * User: George Sun
 * Date: 7/12/13
 * Time: 7:40 PM
 */
public class WeiboModule extends AbstractModule {

    @Override
    protected void configure() {
        bind(WeiboSender.class).to(WeiboBrowserSender.class);
        bind(Shorter.class).to(UrlShortener.class);
    }
}

import com.google.inject.Guice;
import com.google.inject.Injector;

/**
 * Driver class.
 * <p/>
 * User: George Sun
 * Date: 7/12/13
 * Time: 7:44 PM
 */
public class Main {

    public static void main(String[] args) {
        Injector injector = Guice.createInjector(new WeiboModule());
        WeiboClient weiboClient = injector.getInstance(WeiboClient.class);

        String sample = "Sample weibo text here.";
        weiboClient.postWeibo(sample);
    }
}

For complete reference and documents, visit Guice project home.