java的native方法

简单的说, native的方法是Java提供给非Java代码的一个借口, 所以, 使用了native的函数不用实现, 只要一个 ; 就可以了，这个函数就是用非Java代码实现的，例如C，C++等。 下面一个详细的介绍

CONTENTS

• What Is a Native Method?
• Uses for Native Methods
• Benefits and Trade-Offs
• How Does This Magic Work?
• Summary

---

The goal for this chapter is to introduce you to Java's native methods. If you are new to Java, you may not know what native methods are, and even if you are an experienced Java developer, you may not have had a reason to learn more about native methods. At the conclusion of this chapter you should have a better understanding of what native methods are, when and why you may want to use them, and the consequences of using them. You should also have a basic understanding of how native methods work. You will then be more than ready to tackle the next three chapters, which dive into the nitty-gritty details of Java's Native Methods.

What Is a Native Method?

Simply put, a native method is the Java interface to non-Java code. It is Java's link to the "outside world." More specifically, a native method is a Java method whose implementation is provided by non-Java code, most likely C (see Figure 30.1). This feature is not special to Java. Most languages provide some mechanism to call routines written in another language. In C++, you must use the extern "C" stmt to signal that the C++ compiler is making a call to C functions. It is common to see the qualifier pascal in many C compilers to signal that the calling convention should be done in a Pascal convention, rather than a C convention. FORTRAN and Pascal have similar facilities, as do most other languages.

Figure 30.1 : A native method is a Java method whose implementation is provided by non-java code.

In Java, this is done via native methods. In your Java class, you mark the methods you wish to implement outside of Java with the native method modifier-much like you would use the public or static modifiers. Then, rather than supplying the method's body, you simply place a semicolon in its place. As an example, the following class defines a variety of native methods:

public class IHaveNatives
{
native public void Native1( int x ) ;
   native static public long Native2() ;
native synchronized private float Native3( Object o ) ;
   native void Native4( int[] ary ) throws Exception ;
}

This sample class shows a number of possible native methods. As you may have noticed, native methods look much like any other Java method, except a single semicolon is in the place of the method body. Naturally, the body of the method is implemented outside of Java. What you basically define is the interface into this external method. This method declaration describes the Java view of some foreign code.

The only thing special about this declaration is that the keyword native is used as a modifier. Every other Java method modifier can be used along with native, except abstract. This is logical, because the native modifier implies that an implementation exists, and the abstract modifier insists that there is no implementation. Your native methods can be static methods, thus not requiring the creation of an object (or instance of a class). This is often convenient when using native methods to access an existing C-based library. Naturally, native methods can limit their visibility with the public, private, private protected, protected, or unspecified default access. Native methods can also be synchronized (see Chapter 7, "Concurrency and Synchronization"). In the case of a synchronized native method, the Java VM will perform the monitor locking prior to entering the native method implementation code. So, as in Java, the developer is not burdened with doing the actual monitor locking and unlocking.

The example uses a variety (although not all) of types. This is because a native method can be passed any Java type. There is no special procedure within the Java code to pass data to the native method. However, the developer of native methods must be careful that his native methods behave properly when manipulating Java datatypes. Native methods do not undergo the same kinds of checking as a Java method, and they can easily corrupt a Java datatype if care is not taken (see Chapter 31, "The Native Method Interface").

A native method can accept and return any of the Java types-including class types. Of course, the power of exception handling is also available to native methods. The implementation of the native method can create and throw exceptions similar to a Java method. When a native method receives complex types, such as class types (such as Object in the example) or array types (such as the int[] in the example), it has access to the contents of those types. However, the method used to access the contents may vary depending on the Java implementation being used. The major point to remember is that you can access all the Java features from your native implementation code, but it may be implementation-dependent and will surely not be as convenient or easy as it can be done from Java.

The presence of native methods does not affect how other classes call those methods. The caller does not even realize it is calling a native method, so no special code is generated, and the calling convention is the same as for any other method-the calling depends on the method being virtual or static. The Java virtual machine will handle all the details to make the call in the native method implementation. One minor exception may be with the methods marked with the final modifier. The Java implementation may take advantage of a final method and choose to inline its code. It would be doubtful that this could be achieved with a native final method, but this is an optimization issue, not one of functionality. When a class containing native methods is subclassed, the subclass will inherit the native method and also will have the capability of overriding the native method-even with a Java method (that is, the overridden method can be implemented in Java). If a native method is also marked with the final modifier, a subclass is still prevented from overriding it.

Native methods are very powerful, because they effectively extend the Java virtual machine. In fact, your Java code already uses native methods. In the current implementation from Sun, native methods are used in many places to interface to the underlying operating system. This enables a Java program to go beyond the confines of the Java Runtime. With native methods, a Java program can virtually do any application level task.

Uses for Native Methods

Java is a wonderful language to use. However, there are times when you either must interface with existing code, can't express the task in Java, or need the absolute best performance.

Accessing Outside the Java Environment

There are times where a Java application (or applet) must communicate with the environment outside of Java. This is, perhaps, the main reason for the existence of native methods. For starters, the Java implementation will need to communicate with the underlying system. That underlying system may be an operating system such as Solaris or Win32, or it may be a Web browser, or it may be custom hardware, such as a PDA, Set-top-device, and so forth. Regardless of what is under Java, there must be a mechanism to communicate with that system. At some point in a Java program, there will be that point where Java meets the outside world, an interface between Java and non-Java worlds. Native methods provide a simple clean approach to providing this interface without burdening the rest of the Java application with special knowledge.

Accessing the Operating System

The Java virtual machine describes a system that the Java program can rely on to be there. This virtual machine supports the Java Language and its runtime library. It may be composed of an interpreter or can be libraries linked to native code. Regardless of its form, it is not a complete system and often relies on an existing system underneath to provide a lot of support. More than likely, a full-fledged operating system, such as Solaris or Win32, resides beneath it. The use of native methods enables the Java Runtime to be written in Java yet have access to the underlying operating system, or even the parts of the Java virtual machine that may be written in a language such as C. Further, if a Java feature does not encapsulate an operating system feature needed by an application, native methods can be used to access this feature.

Embedded Java

It is conceivable to see a Java virtual machine embedded inside another program. Several WWW browsers come to mind. Perhaps this enclosing program is not implemented in Java. The Java Runtime may need to access the enclosing program for services to support the Java environment. Once again, native methods provide a clean interface for this access to the surrounding program. Furthermore, the vendor of the program may wish to expose some features of the program to a Java applet. The vendor would simply need to create a set of Java classes containing native methods, which provide the interface for the Java application into the program. The native method implementation would then be the "glue" between the Java applet and the internals of the enclosing program.

Custom Hardware

Another important possible application of native methods being used to access a non-Java world is providing Java programs access to custom hardware. Perhaps a Java virtual machine is running within a PDA or Set-Top-Device. A lot of what would normally be in an operating system may exist in hardware or software embedded in ROM, or other custom chip sets. Another possibility is that a computer may be equipped with a dedicated graphics card. It would be ideal to have Java make use of the graphics hardware. A set of Java classes with native methods defined would provide the Java program access to these features.

Sun's Java

In the current implementation from Sun, the Java interpreter is written in C and can thus talk to the outside environment as any normal C program can. A majority of the Java Runtime is written in Java and may make calls into the interpreter or directly to the outside environment, all via native methods. The application deals mostly with the Java Runtime, but it may also talk to the outside environment via native methods. For example in the class java.lang.Thread the setPriority() method is implemented in Java but calls the method setPriority0(), which is a native method in the Thread class. This native method is implemented in C and resides within the Java virtual machine itself. On the Windows 95 platform this native method will then call (eventually) the Win32 SetPriority() API. This is an example where the native method implementation was provided by the Java virtual machine directly. In most cases the native method implementation resides in an external dynamic link library (discussed in a following section), but the call still goes through the Java virtual machine.

Performance

Another major reason for native methods is performance. The Java language trades some performance for features like its dynamic nature, garbage collecting, and safety. Some Java implementations, like the current crop, may be interpreters, which also add extra overhead. The lost performance can be small as the implementation technology for Java systems improve, but until then and even after there may always be a small performance overhead for certain functionality a Java program may need. This functionality can be pushed down into a native method. That native method can then be implemented efficiently at the native lower level of the system on which the Java virtual machine is running. Once at the native implementation level, the developer can use the best-suited language, such as C or even assembler. In this way, maximum performance can be achieved in those specific areas while the bulk of the application is done within the safe and robust Java virtual machine. One area where you may choose to implement some parts of an application in native methods is time-intensive computations, such as graphics rendering, simulation models, and so forth.

Accessing Existing Libraries

The fact that Java is targeted at the production of platform-neutral code means that the current implementations may not access system features that you may need. An example is a database engine. If you need to, you can use the native method facility to provide your own interface to such libraries. Further, you may want to use Java to write applications that use existing in-house libraries. Again, the use of native methods enables you to make such an interface. This enables you to leverage off your existing code base as well as gradually introduce Java-based applications among your other applications coded in an older language.