06 local method interface

What is a local approach ？

• Simply speak , One Native Method It's a Java Call not Java Interface of code . One Native Method It's such a Java Method ： The implementation of this method consists of Java Language implementation , such as C. This feature is not Java Unique to , Many other programming languages have this mechanism , For example C++ in , You can use it. extern "C" inform C++ The compiler calls a C Function of .
• “A native method is a Java method whose implementation is provided by non-java code.”（ The local method is a Java Methods , Its concrete realization is right and wrong Java Code implementation ）
• Defining a native method when , Does not provide an implementer （ It's like defining a Java interface）, Because its realization is caused by non java Language is realized on the outside .
• The function of the local interface is to integrate different programming languages into Java used , Its original intention is to integrate C/C++ Program .

give an example

identifier native Can be compared with other java Identifiers are used together , however abstract With the exception of

public class IHaveNatives {
    
    public native void Native1(int x);

    public native static long Native2();

    private native synchronized float Native3(Object o);

    native void Native4(int[] ary) throws Exception;
    
}

Why use Native Method？

Java It is very convenient to use , However, some levels of tasks use Java It's not easy to implement , Or when we care about the efficiency of the program , Here's the problem .

• And Java Environmental diplomacy is mutual ：
  • Sometimes Java Applications need to be associated with Java The outside environment interacts , This is the main reason for the existence of local methods .
  • You can think about Java Need to work with some underlying systems , In some cases, such as the exchange of hardware or operating system information .
  • The local approach is just such a communication mechanism ： It provides us with a very simple interface , And we don't have to understand Java Trivial details beyond the application .
• Interaction with the operating system ：
  • JVM Supporting Java The language itself and the runtime library , It is Java The platform on which programs live , It has an interpreter （ Interpret bytecode ） And some libraries linked to local code .
  • But anyway , It's not a complete system after all , It often relies on the support of an underlying system . These underlying systems are often powerful operating systems .
  • By using local methods , We can use Java Realized jre Interaction with the underlying system , even to the extent that JVM Part of it is to use C Written .
  • also , If we're going to use some Java When the language itself does not provide encapsulated features of the operating system , We also need to use local methods .
• Sun’s Java
  • Sun The interpreter of C Realized , This makes it look like something ordinary C Interact with the outside as well .jre Mostly with Java Realized , It also interacts with the outside world through some local methods .
  • for example ： class java.lang.Thread Of setPriority( ) The method is to use Java Realized , But it calls the local methods in the class setPriority0( ). This method is local C Realized , And implanted JVM Inside Windows 95 On the platform , This local method will eventually call Win32 setpriority( ) API.
  • This is a concrete implementation of a local method by JVM Provide directly , More often than not, local methods are made up of external DLL （external dynamic link library） Provide , Then be JVM call .

present situation

At present, this method is used less and less , Except for hardware related applications , Such as through Java Program driven printer or Java System management production equipment , It is rare in enterprise applications . Because now the communication between heterogeneous fields is very developed , You can use Socket signal communication , You can also use Web Service wait , No more introductions .