JNI官方教程与规范：Java本地接口详解

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jni.book Page 2 Thursday, February 21, 2002 4:36 PM

CHAPTER 1

Introduction

THE Java™ Native Interface (JNI) is a powerful feature of the Java platform.

Applications that use the JNI can incorporate native code written in programming

languages such as C and C++, as well as code written in the Java programming

language. The JNI allows programmers to take advantage of the power of the Java

platform, without having to abandon their investments in legacy code. Because the

JNI is a part of the Java platform, programmers can address interoperability issues

once, and expect their solution to work with all implementations of the Java plat-

form.

This book is both a programming guide and a reference manual for the JNI.

The book consists of three parts:

• Chapter 2 introduces the JNI through a simple example. It is a tutorial

intended for the beginning users who are unfamiliar with the JNI.

• Chapters 3 to 10 constitute a programmer’s guide that gives a broad overview

of a number of JNI features. We will go though a series of short but descrip-

tive examples to highlight various JNI features and to present the techniques

that have proven to be useful in JNI programming.

• Chapters 11 to 13 present the deﬁnitive speciﬁcation for all JNI types and

functions. These chapters are also organized to serve as a reference manual.

This book tries to appeal to a wide audience with different needs for the JNI.

The tutorial and programming guide are targeted toward beginning programmers,

whereas experienced developers and JNI implementors may ﬁnd the reference

sections more useful. The majority of readers will likely be developers who use

the JNI to write applications. The term “you” in this book will implicitly denote

developers who program with the JNI, as opposed to JNI implementors or end-

users of applications written using the JNI.

The book assumes that you have basic knowledge of the Java, C, and C++

programming languages. If not, you may refer to one of the many excellent books

that are available: The Java™ Programming Language, Second Edition,byKen

Arnold and James Gosling (Addison-Wesley, 1998), The C Programming Lan-

guage, Second Edition, by Brian Kernighan and Dennis Ritchie (Prentice Hall,

jni.book Page 3 Thursday, February 21, 2002 4:36 PM

1.1 The Java Platform and Host Environment INTRODUCTION

1988), and The C++ Programming Language, Third Edition, by Bjarne Strous-

trup (Addison-Wesley, 1997).

The remainder of this chapter introduces the background, role, and evolution

of the JNI.

1.1 The Java Platform and Host Environment

Because this book covers applications written in the Java programming language

as well as in native (C, C++, etc.) programming languages, let us ﬁrst clarify the

exact scope of the programming environments for these languages.

The Java platform is a programming environment consisting of the Java vir-

tual machine (VM) and the Java Application Programming Interface (API).

Java

applications are written in the Java programming language, and compiled into a

machine-independent binary class format. A class can be executed on any Java

virtual machine implementation. The Java API consists of a set of predeﬁned

classes. Any implementation of the Java platform is guaranteed to support the

Java programming language, virtual machine, and API.

The term host environment represents the host operating system, a set of

native libraries, and the CPU instruction set. Native applications are written in

native programming languages such as C and C++, compiled into host-speciﬁc

binary code, and linked with native libraries. Native applications and native librar-

ies are typically dependent on a particular host environment. A C application built

for one operating system, for example, typically does not work on other operating

systems.

Java platforms are commonly deployed on top of a host environment. For

example, the Java Runtime Environment (JRE) is a Sun product that supports the

Java platform on existing operating systems such as Solaris and Windows. The

Java platform offers a set of features that applications can rely on independent of

the underlying host environment.

1.2 Role of the JNI

When the Java platform is deployed on top of host environments, it may become

desirable or necessary to allow Java applications to work closely with native code

written in other languages. Programmers have begun to adopt the Java platform to

build applications that were traditionally written in C and C++. Because of the

As used herein, the phrases “Java virtual machine” or “Java VM” mean a virtual machine for the

Java platform. Similarly, the phrase “Java API” means the API for the Java platform.

jni.book Page 4 Thursday, February 21, 2002 4:36 PM

1.3 Implications of Using the JNI INTRODUCTION

1.3 Implications of Using the JNI

Remember that once an application uses the JNI, it risks losing two beneﬁts of the

Java platform.

First, Java applications that depend on the JNI can no longer readily run on

multiple host environments. Even though the part of an application written in the

Java programming language is portable to multiple host environments, it will be

necessary to recompile the part of the application written in native programming

languages.

Second, while the Java programming language is type-safe and secure, native

languages such as C or C++ are not. As a result, you must use extra care when

writing applications using the JNI. A misbehaving native method can corrupt the

entire application. For this reason Java applications are subject to security checks

before invoking JNI features.

As a general rule, you should architect the application so that native methods

are deﬁned in as few classes as possible. This entails a cleaner isolation between

native code and the rest of the application.

1.4 When to Use the JNI

Before you embark on a project using the JNI, it is worth taking a step back to

investigate whether there are alternative solutions that are more appropriate. As

mentioned in the last section, applications that use the JNI have inherent disadvan-

tages when compared with applications written strictly in the Java programming

language. For example, you lose the type-safety guarantee of the Java program-

ming language.

A number of alternative approaches also allow Java applications to interoper-

ate with code written in other languages. For example:

• A Java application may communicate with a native application through a

TCP/IP connection or through other inter-process communication (IPC)

mechanisms.

• A Java application may connect to a legacy database through the JDBC™

API.

• A Java application may take advantage of distributed object technologies such

as the Java IDL API.

A common characteristic of these alternative solutions is that the Java applica-

tion and native code reside in different processes (and in some cases on different

machines). Process separation offers an important beneﬁt. The address space pro-

jni.book Page 6 Thursday, February 21, 2002 4:36 PM

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