Java Reflection API允許在運(yùn)行時(shí)檢查和操作類、方法、字段等組件，使代碼具備動(dòng)態(tài)適應(yīng)能力。它可用於發(fā)現(xiàn)類結(jié)構(gòu)、訪問(wèn)私有字段、動(dòng)態(tài)調(diào)用方法、創(chuàng)建未知類的實(shí)例，常見於Spring、Hibernate等框架，也用於序列化庫(kù)、測(cè)試工具、插件系統(tǒng)等場(chǎng)景。 1. 依賴注入框架通過(guò)反射實(shí)現(xiàn)自動(dòng)裝配；2. 序列化庫(kù)利用反射讀取對(duì)象字段生成JSON；3. 測(cè)試工具使用反射調(diào)用測(cè)試方法並生成代理；4. 插件系統(tǒng)藉助反射動(dòng)態(tài)加載和執(zhí)行外部類。但需注意性能開銷、安全限制、封裝破壞及編譯期安全缺失等問(wèn)題，應(yīng)謹(jǐn)慎使用以避免不必要的複雜性和錯(cuò)誤。

Java Reflection API lets you inspect and manipulate classes, methods, fields, and other components of a Java program at runtime. It's a powerful tool that gives you the ability to work with code dynamically — even if you don't know exactly what classes or methods are available when you're writing your code.

This capability comes in handy in many real-world scenarios, especially when flexibility and runtime adaptability are needed.

What Can You Do With Reflection?

With the Reflection API, you can:

• Discover class structure (like method names, parameters, return types)
• Access and modify private fields
• Invoke methods dynamically
• Create instances of classes without knowing their names ahead of time

For example, you might load a class by its name from a configuration file, call a method on it based on user input, or access internal state for debugging purposes.

One common use is in frameworks like Spring or Hibernate, which rely heavily on reflection to wire dependencies or map database rows to objects without requiring manual boilerplate code.

Common Use Cases for Java Reflection

1. Dependency Injection Frameworks

Frameworks like Spring use reflection to automatically inject dependencies. When you annotate a field or constructor with @Autowired , Spring uses reflection to find and instantiate the required classes at runtime.

How it works:

• The framework scans for annotated classes
• Uses reflection to inspect constructors or fields
• Instantiates and wires dependencies as needed

Without reflection, this kind of dynamic wiring wouldn't be possible.

2. Serialization and Deserialization Libraries

Libraries such as Jackson or Gson use reflection to convert Java objects into JSON and vice versa. They inspect the object's fields, including private ones, and read/write their values dynamically.

For example:

 MyObject obj = new MyObject();
obj.setName("Test");
String json = gson.toJson(obj); // uses reflection internally

These libraries don't need getters/setters for every field because they can directly access them via reflection.

3. Testing Tools and Mocking Frameworks

Tools like Mockito and JUnit make heavy use of reflection to invoke test methods, inspect annotations like @Test , and mock behavior by dynamically generating proxy classes.

If you've ever used something like:

 when(mockedList.get(0)).thenReturn("first");

That get method is being intercepted using reflection and bytecode manipulation behind the scenes.

4. Plugin Systems and Dynamic Class Loading

Applications that support plugins often load .jar files at runtime and inspect their contents using reflection. This allows the app to discover and run new features without recompilation.

Example flow:

• Load a .jar file
• Get a list of classes inside
• Check if they implement a known interface
• Instantiate and use them accordingly

This pattern is used in IDEs, game mods, and modular enterprise apps.

Things to Keep in Mind When Using Reflection

Reflection is powerful, but not without downsides:

• Performance Overhead : Reflective operations are slower than direct calls.
• Security Restrictions : Some environments restrict reflective access for security reasons.
• Accessing Private Members : While possible, it breaks encapsulation and should be done carefully.
• Compile-Time Safety Lost : Mistakes like wrong method names won't show up until runtime.

Also, always remember to set setAccessible(true) if you're accessing private fields or methods, otherwise you'll get IllegalAccessException.

Reflection isn't something you reach for every day, but when you need it — like when building generic libraries or tools — it's indispensable. Just keep it focused and avoid overusing it unless necessary.

基本上就這些。

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