The application of dynamic proxy in Android

Dynamic proxy is a very useful design pattern in programming, which allows you to create a proxy object at runtime instead of the original object to perform additional logic before and after method calls. In Android development, dynamic proxies can be used for various use cases such asperformance monitoring, AOP (aspect-oriented programming), and event handling. This article will delve into the principles, uses, and practical examples of Android dynamic proxies.

What is a dynamic proxy?

Dynamic proxying is a technique that creates a proxy object in place of the original object to perform additional operations before and after method invocations. Proxy objects usually implement the same interface as the original object, but can add custom behavior. A dynamic proxy is generated at runtime, so it does not need to know the type of the original object at compile time.

The principle of dynamic proxy

The principle of dynamic proxy involves two key parts:

1. InvocationHandler: This is an interface, usually implemented by developers. It contains a method invoke that is called when the method on the proxy object is called. Within the invoke method, you can define logic that is executed before and after the method is called.
2. Proxy (proxy class): This is a class provided by Java for creating proxy objects. You need to pass a ClassLoader, a set of interfaces, and a InvocationHandler to the Proxy.newProxyInstance method, and it will generate the proxy object.

Here is a sample code that demonstrates how to create a simple dynamic proxy:

import java.lang.reflect.InvocationHandler
import java.lang.reflect.Method
import java.lang.reflect.Proxy

// 接口
interface MyInterface {
    
    
    fun doSomething()
}

// 实现类
class MyImplementation : MyInterface {
    
    
    override fun doSomething() {
    
    
        println("Original method is called.")
    }
}

// 调用处理器
class MyInvocationHandler(private val realObject: MyInterface) : InvocationHandler {
    
    
    override fun invoke(proxy: Any, method: Method, args: Array<Any?>?): Any? {
    
    
        println("Before method is called.")
        val result = method.invoke(realObject, *(args ?: emptyArray()))
        println("After method is called.")
        return result
    }
}

fun main() {
    
    
    val realObject = MyImplementation()
    val proxyObject = Proxy.newProxyInstance(
        MyInterface::class.java.classLoader,
        arrayOf(MyInterface::class.java),
        MyInvocationHandler(realObject)
    ) as MyInterface

    proxyObject.doSomething()
}

Running the above code will output:

Before method is called.
Original method is called.
After method is called.

Here, MyInvocationHandler intercepts the call to the doSomething method and adds additional logic before and after the method.

Dynamic proxy in Android

In Android, dynamic proxies are usually implemented using Java'sjava.lang.reflect.Proxy classes. This class allows you to create a proxy object that implements a specified interface and can intercept calls to interface methods to perform additional logic. In Android development, common uses include performance monitoring, permission checking, logging, and event handling.

The purpose of dynamic proxy

Performance monitoring

You can use dynamic proxies to monitor method execution times to analyze application performance. For example, you can create a performance monitoring agent that records the current time before each method call and then calculates the execution time after the method call.

import android.util.Log

class PerformanceMonitorProxy(private val target: Any) : InvocationHandler {
    
    
    override fun invoke(proxy: Any, method: Method, args: Array<Any?>?): Any? {
    
    
        val startTime = System.currentTimeMillis()
        val result = method.invoke(target, *(args ?: emptyArray()))
        val endTime = System.currentTimeMillis()
        val duration = endTime - startTime
        Log.d("Performance", "${method.name} took $duration ms to execute.")
        return result
    }
}

AOP (Aspect Oriented Programming)

Dynamic proxy is also one of the core concepts of AOP. AOP allows you to separate cross-cutting concerns (such as logging, transaction management, and security checks) from business logic to better maintain and extend your code. You can apply these concerns to multiple classes and methods by creating appropriate proxies.

event handling

Android often needs to handle various events on the user interface, such as click events, sliding events, etc. You can use dynamic proxies to simplify event processing code and separate event processing logic from Activity or Fragment to make the code more modular and maintainable.

Practical example

Here is a simple example that demonstrates how to use dynamic proxies in Android to handle click events:

import android.util.Log
import java.lang.reflect.InvocationHandler
import java.lang.reflect.Method
import java.lang.reflect.Proxy
import android.view.View

class ClickHandlerProxy(private val target: View.OnClickListener) : InvocationHandler {
    
    
    override fun invoke(proxy: Any, method: Method, args: Array<Any?>?): Any? {
    
    
        if (method.name == "onClick") {
    
    
            Log.d("ClickHandler", "Click event intercepted.")
            // 在事件处理前可以执行自定义逻辑
        }
        return method.invoke(target, *args.orEmpty())
    }
}

// 使用示例
val originalClickListener = View.OnClickListener {
    
    
    // 原始的点击事件处理逻辑
}

val proxyClickListener = Proxy.newProxyInstance(
    originalClickListener::class.java.classLoader,
    originalClickListener::class.java.interfaces,
    ClickHandlerProxy(originalClickListener)
) as View.OnClickListener

button.setOnClickListener(proxyClickListener)

This way, you can execute custom logic before and after the original click event handling logic without modifying the original OnClickListener implementation.

in conclusion

Dynamic proxies are one of the most powerful tools in Android development, allowing you to add additional behavior without modifying the original object. Dynamic agents are widely used in performance monitoring, AOP and event processing. With a deep understanding of the principles and uses of dynamic proxies, you can better design and maintain Android applications.

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