# OSGi service proxy
> The OSGi Alliance, "[OSGi Core Release 6](https://docs.osgi.org/download/r6/osgi.core-6.0.0.pdf)", June 2014, pp 385-387.
> [OSGi Core Release 7 (55.3.1 Proxying)](https://docs.osgi.org/specification/osgi.core/7.0.0/framework.servicehooks.html#d0e45668)
###### tags: `張宸翊` `OM2M`

## ServiceTracker
功能 : 對服務做追蹤，例如服務何時被註冊、何時被移除、何時被修改等。
* ```addingService``` 服務註冊時
* ```modifiedService``` 服務修改時
* ```removedService``` 服務移除時

Example ([完整程式碼link](https://hackmd.io/G2fwvsJ8Q76-HSEdfJKtPA#Activatorjava))

ServiceTracker<Object, Object>(bundleContext, CseService.class.getName(), null) {
	public void removedService(ServiceReference<Object> reference, Object service) {
		try {
			adapter.setCse(null);
		} catch (IllegalArgumentException e) {
			System.out.println("Error removing SclService");
		}
	}

	public Object addingService(ServiceReference<Object> reference) {
		CseService cse = (CseService) this.context.getService(reference);
		try {
			adapter.setCse(cse);
		} catch (Exception e) {
			System.out.println("Error adding SclService");
		}
		return cse;
	}
};

## OSGi service hook
A kind of OSGi service.
* ```EventListenerHook``` 服務註冊、移除、修改時觸發
* ```FindHook``` 服務請求時觸發
* ```ListenerHook``` 服務監聽增加或刪除時觸發

> [參考連結](https://docs.osgi.org/specification/osgi.core/7.0.0/framework.servicehooks.html)

## Proxying
Proxying an existing service for a specific bundle requires the following steps:
* Hide the existing service $X$
* Hiding service X can be implemented with a combination of the **Event Listener Hook** and the **Find Hook**.
* Register a proxy $X'$ with the same properties as $X$

1. **Event Listener Hook** : It can be used to hide any service events from the target bundle.
2. **Find Hook** : It can be used to remove $X$ from the delivered results of the ```getServiceReference(s)``` methods.
* remove only
* The **Event Listener Hook** and **Find Hook** both allow the interceptor to remove elements from a collection and not add elements.

Bundle A directly uses Service $X$, in the proxying case, the Proxy Bundle hides the original and provides an alternative.


## 範例程式

### 開發環境
| | version |
| ---- |:------------------------- |
| OS | windows 10 2004 19041.264 |
| JAVA | 1.8.0_251 |
| IDE | Eclipse 2019-06 (4.12.0) |

### 說明
:::info
目前有一個Bundle B，提供了一個Service X給Bundle A
我們希望原始Service X不要讓Bundle A拿到，而是取得Service X'(實際名稱與Service X一樣，用X'只是比較好理解)
:::
### 1. Create a Plug-in Project
先建立一個新的project
「File → New → Project...」

接著選「Plug-in Development → Plug-in Project → Next」



* 重複以上步驟，Project name填入不同的名稱，產生三個project (ServiceProxy_TEST、BundleA、BundleB)

### 2-1. BundleA

:::info
Bundle A 希望取得 ServiceX 來用
:::
:::spoiler BundleA > Activator.java

package bundlea;

import bundleb.ServiceX;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;

public class Activator implements BundleActivator {

	private static BundleContext context;

	static BundleContext getContext() {
		return context;
	}

	public void start(BundleContext bundleContext) throws Exception {
		System.out.println("Bundle A start");
		Activator.context = bundleContext;
		ServiceReference<?> reference = context.getServiceReference(ServiceX.class.getName());
		if(null != reference){
			ServiceX service = (ServiceX) context.getService(reference);
			if(null != service){
				System.out.println("Services From "+service.SayHello("Bundle A"));
			}
		} else {
			System.out.println(bundleContext.getBundle().getSymbolicName()+" 無法取得服務");
		}
	}

	public void stop(BundleContext bundleContext) throws Exception {
		Activator.context = null;
	}

}

:::
:::spoiler BundleA > MANIFEST.MF

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: BundleA
Bundle-SymbolicName: BundleA
Bundle-Version: 1.0.0.qualifier
Bundle-Activator: bundlea.Activator
Bundle-RequiredExecutionEnvironment: JavaSE-1.8
Automatic-Module-Name: BundleA
Import-Package: bundleb,
 org.osgi.framework;version="1.3.0",
 org.osgi.util.tracker;version="1.5.2"
Bundle-ActivationPolicy: lazy

:::

### 2-2. BundleB

* bundleb右鍵 → New → File → File name : HelloNCKU.java → Finish
* bundleb右鍵 → New → File → File name : ServiceX.java → Finish
:::info
Bundle B 提供了一個 Service X (HelloNCKU.java)，我們立用 2-3 中的 FindHook 將它取得的 Service X (HelloNCKU.java)，改為 Service X (HelloDCNLab.java)
:::
:::spoiler BundleB > Activator.java

package bundleb;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceRegistration;

public class Activator implements BundleActivator {

	private static BundleContext context;

	static BundleContext getContext() {
		return context;
	}

	public void start(BundleContext bundleContext) throws Exception {
		Activator.context = bundleContext;
		ServiceRegistration<?> reference = bundleContext.registerService(ServiceX.class, new HelloNCKU(), null);
		System.out.println("Bundle B 提供一個 Service X");
		System.out.println(reference);
	}

	public void stop(BundleContext bundleContext) throws Exception {
		Activator.context = null;
	}

}

:::
:::spoiler BundleB > HelloNCKU.java

package bundleb;

public class HelloNCKU implements ServiceX{

	@Override
	public String SayHello(String name) {
		// TODO Auto-generated method stub
		System.out.printf("Helle NCKU %s\n",name);
		return this.getClass().getName();
	}
	
}

:::
:::spoiler BundleB > ServiceX.java

package bundleb;

public interface ServiceX {
	public String SayHello(String name);
}

:::
:::spoiler BundleB > MANIFEST.MF

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: BundleB
Bundle-SymbolicName: BundleB
Bundle-Version: 1.0.0.qualifier
Bundle-Activator: bundleb.Activator
Bundle-RequiredExecutionEnvironment: JavaSE-1.8
Automatic-Module-Name: BundleB
Import-Package: org.osgi.framework;version="1.3.0",
 org.osgi.util.tracker;version="1.5.2"
Bundle-ActivationPolicy: lazy
Export-Package: bundleb

:::

### 2-3. ServiceProxy_TEST

* serviceproxy_test右鍵 → New → File → File name : HelloDCNLab.java → Finish
* serviceproxy_test右鍵 → New → File → File name : ProxyServiceX.java → Finish
:::info
當 Bundle B 希望取得 Service X 時，find會被呼叫，在這個時候，將原始的 Service X (HelloNCKU.java) 移除，Bundle B 就無法取得原本的 Service X (HelloNCKU.java)，改取得 Service X (HelloDCNLab.java)
:::
:::spoiler ServiceProxy_TEST > Activator.java

package serviceproxy_test;

import java.util.Dictionary;
import java.util.Hashtable;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceRegistration;

import bundleb.ServiceX;

public class Activator implements BundleActivator {

	private static BundleContext context;

	static BundleContext getContext() {
		return context;
	}

	public void start(BundleContext bundleContext) throws Exception {
		Activator.context = bundleContext;
		HelloDCNLab h = new HelloDCNLab();
		Dictionary<String,Object>properties = new Hashtable<String,Object>();
		properties.put("proxied","true");
		ServiceRegistration<?> reference = bundleContext.registerService(ServiceX.class, h, properties);
		System.out.println("ServiceProxy_TEST 提供一個 Service X");
		System.out.println(reference);
		ProxyServiceX PS = new ProxyServiceX(context, "BundleA", "BundleB");
		PS.open();
	}

	public void stop(BundleContext bundleContext) throws Exception {
		Activator.context = null;
	}

}

:::
:::spoiler ServiceProxy_TEST > HelloDCNLab.java

package serviceproxy_test;

import bundleb.ServiceX;

public class HelloDCNLab implements ServiceX{

	@Override
	public String SayHello(String name) {
		// TODO Auto-generated method stub
		System.out.printf("Helle DCNLab %s\n",name);
		return this.getClass().getName();
	}
	
}

:::
:::spoiler ServiceProxy_TEST > ProxyServiceX.java

package serviceproxy_test;

import java.util.Collection;
import java.util.Iterator;

import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;
import org.osgi.framework.ServiceRegistration;
import org.osgi.framework.hooks.service.FindHook;

public class ProxyServiceX implements FindHook {
	
	final BundleContext context;
	final String FindBundleName, removeFrom;

	public ProxyServiceX(BundleContext context, String FindBundleName, String removeFrom) {
		this.context = context;
		this.FindBundleName = FindBundleName;
		this.removeFrom = removeFrom;
	}
	
	public void open() {
		// TODO Auto-generated method stub
		context.registerService(new String[]{FindHook.class.getName()},this,null);
	}

	/*
	 * find 是 FindHook需要實作的function，當有其他bundle使用getServiceReference，find會被osgi呼叫
	 * Once registered, these services will receive their event callbacks. 
	 * In the find hook, the target Service Reference is removed from the results 
	 * if the bundle that called the getServiceReference(s) method is the target bundle.
	 */
	@Override
	public void find(BundleContext Context, String arg1, String arg2, boolean arg3, Collection<ServiceReference<?>> references) {
		// TODO Auto-generated method stub
		if(Context.getBundle().getSymbolicName().equals(FindBundleName)) {
			System.out.println("============= [FindHook find] =============START");
			Iterator<ServiceReference<?>> iterator = references.iterator();
			while (iterator.hasNext()) {
				ServiceReference<?> sr = (ServiceReference<?>) iterator.next();
				System.out.println("[find] "+FindBundleName +" getService : "+ sr);
				System.out.println("This service is from : "+sr.getBundle().getSymbolicName());
				if (sr.getBundle().getSymbolicName().equals(removeFrom)) {
                    iterator.remove();
                    System.out.println("remove " + sr);
                } else {
                	System.out.println("do not remove " + sr);
                }
			}
			System.out.println("============= [FindHook find] =============END");
		}
	}

}

:::

:::spoiler ServiceProxy_TEST > MANIFEST.MF

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: ServiceProxy_TEST
Bundle-SymbolicName: ServiceProxy_TEST
Bundle-Version: 1.0.0.qualifier
Bundle-Activator: serviceproxy_test.Activator
Bundle-RequiredExecutionEnvironment: JavaSE-1.8
Automatic-Module-Name: ServiceProxy_TEST
Import-Package: bundleb,
 org.osgi.framework;version="1.3.0",
 org.osgi.framework.hooks.service;version="1.1.0"
Bundle-ActivationPolicy: lazy

:::
### 3-1. 測試執行
1. Run → RunConfigurations
2. 先 Deselect All → 然後將剛剛新增的那三個bundle打勾 → Add Required Bundles → 設定Start Level
3. 將 org.eclipse.equinox.console、org.apache.felix.gogo.shell 也打勾
4. 最後 Apply → Run
5. Result

### 3-2. 測試執行 - 不使用Service Proxy
6. 將ServiceProxy_TEST > Activator.java line 29 註解
7. (如果有做3-1 step1~4) Run → Run As → OSGi Framework
8. Result

---

[範例程式碼 2020/06/10](https://drive.google.com/file/d/1h0DSCqX7lag8Z07IDgPNEBFd8sbf8xoM/view?usp=sharing)

> 補充參考資料
[OSGi 系列（七）之服务的监听、跟踪、声明等](https://www.cnblogs.com/binarylei/p/8542060.html)
[OSGi Service Hook to Log All Service Invocations Using Dynamic Proxy](https://dzone.com/articles/osgi-service-hook-log-all)
[OSGi Core Release 7 (55.3.1 Proxying)](https://docs.osgi.org/specification/osgi.core/7.0.0/framework.servicehooks.html#d0e45668)