A bridge method

本文探讨了Java泛型中桥方法的概念及其作用,特别是在参数类型转换和协变覆盖的情况下如何解决类继承带来的问题。通过实例说明了桥方法如何由编译器自动生成以确保正确执行。

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A bridge method may be created by the compiler when extending a parameterized type whose methods have parameterized arguments.

You can find in this class BridgeMethodResolver a way to get the actual Method referred by a 'bridge method'.

See Create Frame, Synchronize, Transfer Control:

As an example of such a situation, consider the declarations:

class C<T> { abstract T id(T x); } 
class D extends C<String> { String id(String x) { return x; } } 

Now, given an invocation

C c = new D(); 
c.id(new Object()); // fails with a ClassCastException 

The erasure of the actual method being invoked, D.id(String) differs in its signature from that of the compile-time method declaration, C.id(Object). The former takes an argument of type String while the latter takes an argument of type Object. The invocation fails with a ClassCastException before the body of the method is executed.

Such situations can only arise if the program gives rise to an unchecked warning (§5.1.9).

Implementations can enforce these semantics by creating bridge methods. In the above example, the following bridge method would be created in class D:

Object id(Object x) { return id((String) x); } 

This is the method that would actually be invoked by the Java virtual machine in response to the call c.id(new Object()) shown above, and it will execute the cast and fail, as required.

See also Bridge:

as mentioned in the comment, bridge methods are also needed for covariant overriding:

  • In Java 1.4, and earlier, one method can override another if the signatures match exactly.
  • In Java 5, a method can override another if the arguments match exactly but the return type of the overriding method, if it is a subtype of the return type of the other method.

Typically, a method Object clone() can be overridden by a MyObject clone(), but a bridge method will be generated by the compiler:

public bridge Object MyObject.clone(); 
### Bridge2 封装的实现方式 Bridge2 的封装通常用于前端与原生应用之间的通信,其核心思想在于通过统一接口的方式简化跨平台调用逻辑。以下是关于 Bridge2 封装的具体实现方法: #### 一、桥接设计的核心概念 桥梁模式的设计目标是分离抽象部分和其实现部分,使两者可以独立变化[^1]。对于 Bridge2 来说,它主要涉及两种功能: - **前端调用客户端方法** - **客户端调用前端方法** 这些功能可以通过将双方需要暴露的方法挂载到 `window` 对象下的特定命名空间来实现,比如 `window.Bridge2`。 --- #### 二、具体实现方案 ##### 前端调用客户端方法 为了允许前端代码调用客户端的功能模块,可以在客户端初始化阶段注册一组 API 到 `window.Bridge2` 中。例如,在 JavaScript 环境下定义如下结构: ```javascript // 客户端注入的API if (typeof window.Bridge2 === 'undefined') { window.Bridge2 = {}; } window.Bridge2.callNativeMethod = function(methodName, params, callback) { // 调用本地方法的逻辑 console.log(`Calling native method: ${methodName} with parameters`, params); if (callback && typeof callback === 'function') { setTimeout(() => { // 模拟异步回调 const result = { success: true, data: "Response from Native" }; callback(result); }, 500); } }; ``` 此函数接受三个参数:方法名 (`methodName`)、传递给该方法的数据 (`params`) 和可选的回调函数 (`callback`)。 --- ##### 客户端调用前端方法 为了让客户端能够主动触发前端的行为,同样需要在 `window.Bridge2` 上提供相应的入口点。例如: ```javascript window.Bridge2.onMessageFromNative = function(callback) { if (typeof callback !== 'function') throw new Error('Callback must be a valid function'); document.addEventListener('nativeEvent', callback); }; document.dispatchEvent(new CustomEvent('nativeEvent', { detail: { message: 'Hello from Native!' } })); ``` 这里利用了自定义事件机制(Custom Events),使得当有消息来自客户端时,会自动分发至监听器。 --- #### 三、Webview 特殊场景支持 如果是在 Webview 下运行,则可能还需要考虑浏览器环境差异性处理。某些情况下需特别注意 Chrome WebView 的兼容写法[^3]: ```javascript try { window.chrome.webview.postMessage({ action: 'testAction' }); window.chrome.webview.addEventListener('message', event => { console.log('Received message:', event.data); }); } catch(e){ console.error('Chrome webview not supported.'); } ``` 以上片段展示了如何向宿主发送数据以及接收响应的过程。 --- ### 总结 综上所述,Bridge2 的封装实现了双向通讯能力,并且具备良好的扩展性和灵活性。无论是标准网页还是嵌入式的 WebView 场景都能很好地适配。最终形成了一套简单易懂又高效稳定的解决方案。
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