JNI (Java Native Interface) 是一個允許 Java 代碼與用其他語言（如 C、C 或匯編）編寫的本地應(yīng)用程序或庫交互的框架。1. 它的主要作用是作為 Java 與本地代碼之間的橋梁，使 Java 能夠安全地突破 JVM 的隔離限制訪問系統(tǒng)資源；2. 使用場景包括調(diào)用高性能數(shù)學(xué)庫、對接平臺特定 API 或封裝原生 SDK；3. 使用步驟包括聲明 native 方法、生成 C/C 頭文件、實現(xiàn)并編譯為共享庫、在 Java 中加載該庫；4. 注意事項包括數(shù)據(jù)類型差異需使用 JNI 類型、操作 Java 對象需通過 JNI 函數(shù)、調(diào)試復(fù)雜性和降低可移植性等問題。

Java Native Interface (JNI) is a framework that allows Java code to interact with native applications or libraries written in other languages like C, C , or assembly. It’s especially useful when you need to access system-specific resources or legacy code that isn’t available in Java.

What JNI Does

The main job of JNI is to act as a bridge between Java and native code. Java runs inside the JVM (Java Virtual Machine), which isolates it from directly interacting with the operating system or hardware. JNI gives developers a way to break through that isolation safely.

For example:

• You might use JNI to call a high-performance math library written in C for scientific computing.
• Or interface with platform-specific APIs on Windows, Linux, or macOS that don’t have Java wrappers yet.

It works by letting Java classes load native libraries (like .dll files on Windows or .so on Linux), and then declare native methods — methods implemented outside the JVM but callable from Java.

How to Use JNI in Practice

If you want to write JNI code yourself, here are the basic steps:

• Write a Java class with native method declarations.
• Compile that class into bytecode.
• Generate a C/C header file using javah (or newer tools via javac -h).
• Implement the native functions in C/C .
• Compile the native code into a shared library.
• Load the library in your Java app using System.loadLibrary().

One thing to watch out for: data types don’t always match up one-to-one between Java and C/C . For example, Java's int is always 32 bits, while in C it can vary depending on the platform. So you’ll often use specific types like jint, jobject, etc., defined in JNI headers to make sure everything lines up correctly.

Also, accessing Java objects from native code requires careful handling. You can’t just manipulate Java objects directly — you need to use special JNI functions to get and update their fields or call methods.

When and Why You’d Use JNI

You won’t need JNI every day, but there are clear situations where it makes sense:

• Your application needs to interface with hardware or OS features not exposed through Java standard libraries.
• You have performance-critical code already optimized in C/C and don’t want to rewrite it.
• You're building a Java library that wraps a native SDK provided by another vendor.

However, JNI adds complexity. Debugging crashes becomes harder because if your native code segfaults, it takes down the whole JVM. Also, your code becomes less portable — you may need separate builds for each platform.

Still, for many large-scale Java applications — especially games, enterprise software, or embedded systems — JNI is an essential tool for integrating with the rest of the software ecosystem.

So that’s JNI in a nutshell — it connects Java and native code, opens up powerful integration options, but comes with some trade-offs. Not something you use lightly, but definitely valuable when you really need it.

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