Apache xbean :: ASM Shaded (Repackaged) in the Java Library
Apache XBean is an open source Java class library that is used to perform various operations in Java applications, including instantiated objects, calling methods and access attributes.It provides powerful operating capabilities by using ASM Shaded (that is, ASM, which is re -packaged).The following will be introduced in detail the role of Apache Xbean and ASM Shaded in the Java library and provides related Java code examples.
Apache XBean is a toolkit for operating Java class, which provides methods for creating, modifying and processing.ASM Shaded is a lightweight library that is used to operate and modify the Java class at the bytecode level.XBean uses ASM Shaded to analyze and modify the byte code of the Java class, so as to dynamically generate and control instances during runtime.
The following is an example that shows how to use Apache Xbean and ASM Shaded to create a simple Java class and dynamically call it:
First, we need to add the dependencies of Apache Xbean and ASM Shaded.You can add dependencies through maven, as shown below:
<dependencies>
<dependency>
<groupId>org.apache.xbean</groupId>
<artifactId>xbean-asm-shaded</artifactId>
<version>4.14</version>
</dependency>
</dependencies>
Next, we create a simple interface and implementation class:
public interface Greeting {
void sayHello();
}
public class EnglishGreeting implements Greeting {
@Override
public void sayHello() {
System.out.println("Hello!");
}
}
Now, we will use Apache Xbean and ASM Shaded to create a new class dynamically. This class will implement the Greeting interface and cover the Sayhello method:
import org.apache.xbean.asm.shaded.commons.asm.ClassWriter;
import org.apache.xbean.asm.shaded.commons.asm.MethodVisitor;
import org.apache.xbean.asm.shaded.commons.asm.Opcodes;
import org.apache.xbean.asm.shaded.commons.asm.Type;
public class DynamicClassGenerator {
public static void main(String[] args) throws Exception {
// Create a ClassWriter object to generate a new class byte code
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
// Definition category access decoration, name and parent
cw.visit(Opcodes.V1_8, Opcodes.ACC_PUBLIC + Opcodes.ACC_SUPER, "DynamicGreeting", null, Type.getInternalName(Object.class), new String[] {Type.getInternalName(Greeting.class)});
// Define the unspeak -constructor function
MethodVisitor constructor = cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null);
constructor.visitCode();
constructor.visitVarInsn(Opcodes.ALOAD, 0);
constructor.visitMethodInsn(Opcodes.INVOKESPECIAL, Type.getInternalName(Object.class), "<init>", "()V", false);
constructor.visitInsn(Opcodes.RETURN);
constructor.visitMaxs(1, 1);
constructor.visitEnd();
// Define the SAYHELLO method
MethodVisitor method = cw.visitMethod(Opcodes.ACC_PUBLIC, "sayHello", "()V", null, null);
method.visitCode();
method.visitFieldInsn(Opcodes.GETSTATIC, Type.getInternalName(System.class), "out", Type.getDescriptor(PrintStream.class));
method.visitLdcInsn("Hello from dynamic class!");
method.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(PrintStream.class), "println", "(Ljava/lang/String;)V", false);
method.visitInsn(Opcodes.RETURN);
method.visitMaxs(2, 1);
method.visitEnd();
// Get the generated bytecode and load the class
byte[] byteCode = cw.toByteArray();
DynamicClassLoader classLoader = new DynamicClassLoader();
Class<?> dynamicClass = classLoader.defineClass("DynamicGreeting", byteCode);
// Create an instance and call the method
Greeting greeting = (Greeting) dynamicClass.getDeclaredConstructor().newInstance();
greeting.sayHello();
}
}
class DynamicClassLoader extends ClassLoader {
public Class<?> defineClass(String name, byte[] byteCode) {
return defineClass(name, byteCode, 0, byteCode.length);
}
}
Run the above code, and will output "Hello from Dynamic Class!", Indicating that we have successfully created and called an instance of a class with Apache Xbean and ASM Shaded.
Summary: Apache Xbean and ASM Shaded are powerful Java class libraries that can help us dynamically generate and operate the Java class at runtime.By using Xbean and ASM Shaded, we can implement advanced functions such as dynamic proxy and AOP.Through the above examples, we understand how to use Apache Xbean and ASM Shaded to create and call dynamic generated classes.I hope this article will help you understand the role of Apache Xbean and ASM Shaded in the Java library.
