字段：删除

预期目标

假如有一个HelloWorld类，代码如下：

public class HelloWorld {
    public int intValue;
    public String strValue;
}

我们想实现的预期目标：将HelloWorld类的strValue字段删除。

编码实现

import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.FieldVisitor;

public class ClassRemoveFieldVisitor extends ClassVisitor {
    private final String fieldName;
    private final String fieldDesc;

    public ClassRemoveFieldVisitor(int api, ClassVisitor cv, String fieldName, String fieldDesc) {
        super(api, cv);
        this.fieldName = fieldName;
        this.fieldDesc = fieldDesc;
    }

    @Override
    public FieldVisitor visitField(int access, String name, String descriptor, String signature, Object value) {
        if (name.equals(fieldName) && descriptor.equals(fieldDesc)) {
            return null;
        }
        return super.visitField(access, name, descriptor, signature, value);
    }
}

进行转换

import lsieun.utils.FileUtils;
import org.objectweb.asm.*;

public class HelloWorldTransformCore {
    public static void main(String[] args) {
        String relative_path = "sample/HelloWorld.class";
        String filepath = FileUtils.getFilePath(relative_path);
        byte[] bytes1 = FileUtils.readBytes(filepath);

        //（1）构建ClassReader
        ClassReader cr = new ClassReader(bytes1);

        //（2）构建ClassWriter
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);

        //（3）串连ClassVisitor
        int api = Opcodes.ASM9;
        ClassVisitor cv = new ClassRemoveFieldVisitor(api, cw, "strValue", "Ljava/lang/String;");

        //（4）结合ClassReader和ClassVisitor
        int parsingOptions = ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES;
        cr.accept(cv, parsingOptions);

        //（5）生成byte[]
        byte[] bytes2 = cw.toByteArray();

        FileUtils.writeBytes(filepath, bytes2);
    }
}

总结