Talk about the concept of Native and method area in JVM

Directory

1. What is JVM Native

2. How to use JVM Native

3. What is the method area of JVM

4. Practical application of JVM method area

1. What is JVM Native

Native of JVM refers to a mechanism provided by JVM that allows Java programs to access resources and functions of the local system by calling native methods. Native methods are methods written in a language other than Java, usually in C, C++, or assembly language.

In the JVM, the Native method can be declared as native and does not need to implement any Java code, but it needs to use JNI (Java Native Interface) technology to connect the Java program with the native method library. JNI provides a set of standard interfaces so that Java programs can call native methods, and also provides native method interfaces so that native method libraries can access data and objects of Java programs.

Using the Native mechanism can easily interact with the Java program and the underlying system, such as accessing files, executing Shell commands, operating the network, and so on. But since native methods are not subject to the security restrictions of the JVM, extra care is required to avoid possible security holes and memory errors.

The advantages of the JVM Native mechanism mainly include the following aspects:

  1. Ability to access local system resources and underlying functionality: Java is a cross-platform programming language, but some operations require direct access to underlying system resources and functionality. Using the Native mechanism can easily interact with the Java program and the local system, such as accessing files, executing Shell commands, operating the network, and so on.

  2. Improve program performance: Native methods are usually faster than Java methods because they are written in a non-Java language and do not need to be affected by JVM’s interpretation and dynamic optimization. This makes the Native method more advantageous when performing intensive calculations and processing large amounts of data.

  3. Provide a flexible programming experience: Using the Native mechanism can expand the capabilities of the Java platform and interact efficiently with the local system, which allows Java programmers to add custom functions and libraries to applications, and also provides more programming options and language selection.

  4. Provide system-level control: Native methods are not subject to the security restrictions of the JVM, which means they can directly access system-level resources and functions, such as memory, file system, network, etc. These functions are essential for system-level development.

In short, the Native mechanism of the JVM provides a powerful interface for Java programs, which can expand the capabilities of the Java platform and efficiently interact with the local system. The JVM Native mechanism provides a flexible programming experience and can extend the capabilities of the Java platform, while also providing higher performance and system-level control.

2. How to use JVM Native

The Native mechanism of the JVM needs to use JNI (Java Native Interface) technology to connect the Java program with the local method library. Here are some basic steps:

  1. Declare the native method: declare the native method in the Java code, and do not need to implement any Java code, for example:
public native void helloWorld();

  1. Compile Java code to generate .class files;

  2. Generate header files: Use the javah command to generate header files containing native methods, for example:

javah -jni com.example.HelloWorld

This command will generate a header file named com_example_HelloWorld.h in the current directory.

  1. Realize the native method: Write C or C++ code to realize the native method, and use the function prototype declared in the header file, for example:
JNIEXPORT void JNICALL Java_com_example_HelloWorld_helloWorld(JNIEnv *env, jobject obj) {
    printf("Hello World!\
");
}
  1. Compile native method: compile C or C++ code into a dynamic link library (DLL or SO), for example:
gcc -shared -I${JAVA_HOME}/include -I${JAVA_HOME}/include/linux -o libhelloworld.so HelloWorld.c
  1. Run the Java program: add the generated dynamic link library to the Java classpath, and run the Java program, for example:
java -Djava.library.path=.com.example.HelloWorld

Among them -Djava.library.path=. Specifies the path where the local method library is located.

Through the above steps, we can call the local method and access the resources and functions of the local system. Of course, in actual use, issues such as security and compatibility also need to be considered.

3. What is the method area of JVM

The Method Area of the JVM is a memory area used to store data such as class information, constants, and static variables. It is one of the memory areas defined in the Java virtual machine specification, together with the Java heap, virtual machine stack, local method stack, and program counter, it forms the runtime data area of the Java virtual machine.

The method area is shared by threads, that is, all threads can access it, and its life cycle is consistent with the life cycle of the virtual machine. When the JVM starts, the method area is allocated in the heap memory, but it is different from the Java heap in that its size can be fixed or expandable.

Some important metadata are stored in the method area, including the following aspects:

  1. Class information: including class name, parent class name, interface list, field and method descriptors and other information.

  2. Constant pool: used to store literals and symbol references generated during compilation.

  3. Static Variables: Static variables stored on class level.

  4. Method bytecode: the compiled Java method code.

Because the method area stores metadata, it usually does not need to be garbage collected, but as the running time of the application increases, memory overflow may occur in the method area, so a certain degree of memory management is required.

4. Practical application of JVM method area

The JVM method area is mainly used to store data such as class information, constants, and static variables. The more important usage scenarios include the following aspects:

  1. Type information cache: When a Java program needs to use a class, it first needs to find the type information of the class in the method area. If the class has already been loaded, it can be obtained from the cache, avoiding unnecessary access to disk.

  2. String constant pool: String constants in Java are stored in the method area because they are immutable and can be shared by multiple objects. The string constant pool can help us save memory space and improve program operating efficiency.

  3. Static variable storage: All variables declared as static are stored in the method area, and these variables can be accessed and modified throughout the application life cycle.

  4. Dynamic proxy: Dynamic proxy is a very common design pattern. When implementing dynamic proxy, it is necessary to generate a proxy class through the reflection mechanism and dynamically load it into the JVM. The definition of this proxy class is saved in the method area, and after the proxy class is loaded into the JVM, its bytecode will also be saved in the method area as metadata.

To sum up, the JVM method area plays a very important role in Java programs. It can not only improve program execution efficiency and memory utilization, but also support the implementation of some advanced language features and design patterns. In actual development, we need to have a deep understanding of the JVM method area and use it flexibly according to different application scenarios.