使用ASM工具动态修改类

下面这个是用 ASM 工具为 Student 类添加一个 public String 类型的 address 属性：

1，需要添加属性的原始类：Student.java

Java code

public class Student {
    private int age;
    private String name;
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }    
}

2，添加属性的适配器：AddFieldAdapter.java

Java code

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

public class AddFieldAdapter extends ClassAdapter {
    private int accessModifier;
    private String name;
    private String desc;
    private boolean isFieldPresent;

    public AddFieldAdapter(ClassVisitor cv, int accessModifier, String name, String desc) {
        super(cv);
        this.accessModifier = accessModifier;
        this.name = name;
        this.desc = desc;
    }

    @Override
    public FieldVisitor visitField(int access, String name, String desc,
            String signature, Object value) {
        if (name.equals(this.name)) {
            isFieldPresent = true;
        }
        return cv.visitField(access, name, desc, signature, value);
    }

    @Override
    public void visitEnd() {
        if (!isFieldPresent) {
            FieldVisitor fv = cv.visitField(accessModifier, name, desc, null, null);
            if (fv != null) {
                fv.visitEnd();
            }
        }
        cv.visitEnd();
    }
}

3，添加属性的工具 AddField.java

Java code

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import org.objectweb.asm.ClassAdapter;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;

public class AddField {

    private Class clazz = null;
    private ClassReader cr = null;
    private ClassWriter cw = null;
    private ClassAdapter ca = null;
    private File classFile = null;

    private final static String CLASS_FILE_SUFFIX = ".class";

    public AddField(Class clazz) {
        this.clazz = clazz;
    }

    /**
     * 添加一个 public 的类成员
     * @param fieldName     类成员名
     * @param fieldDesc     类成员类型描述
     */
    public void addPublicField(String fieldName, String fieldDesc) {
        if(cr == null) {
            try {
                cr = new ClassReader(clazz.getCanonicalName());
            } catch (IOException e) {
                e.printStackTrace();
            }
            cw = new ClassWriter(cr, ClassWriter.COMPUTE_MAXS);
        }
        if(ca == null) {
            ca = new AddFieldAdapter(cw, Opcodes.ACC_PUBLIC, fieldName, fieldDesc);
        } else {
            ca = new AddFieldAdapter(ca, Opcodes.ACC_PUBLIC, fieldName, fieldDesc);
        }
    }

    /**
     * 将字节码写入类的 .class 文件
     *
     */
    public void writeByteCode() {
        cr.accept(ca, ClassReader.SKIP_DEBUG);
        byte[] bys = cw.toByteArray();
        OutputStream os = null;
        try {
            os = new FileOutputStream(getFile());
            os.write(bys);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                os.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 获得类文件的 File 对象
     * @return
     */
    private File getFile() {
        if(classFile == null) {
            StringBuffer sb = new StringBuffer();
            sb.append(clazz.getResource("/"))
                .append(clazz.getCanonicalName().replace(".", File.separator))
                .append(CLASS_FILE_SUFFIX);
            classFile = new File(sb.substring(6));
        }
        return classFile;
    }
}

4，字节码处理：PreCompileProcess.java

Java code

public class PreCompileProcess {

    public static void main(String[] args) {

        // 为 Student 添加字段
        AddField add = new AddField(Student.class);

        // 添加一个名为 address，类型为 java.lang.String 的 public 字段 
        add.addPublicField("address", "Ljava/lang/String;");

        // 再增加一个名为 tel，类型为 int 的 public 方法
        add.addPublicField("tel", "I");

        // 重新生成 .class 文件
        add.writeByteCode();
    }
}

5，测试类：Test.java

Java code

public class Test {

    public static void main(String[] args) {
        Student stu = new Student();
        stu.setAge(10);
        stu.setName("Tom");
        stu.address = "Beijing";    // 新增加的 address 字段
        stu.tel = 56;               // 新增加的 tel 字段
        System.out.println("Name:    " + stu.getName());
        System.out.println("Age:     " + stu.getAge());
        System.out.println("Address: " + stu.address);
        System.out.println("Tel:     " + stu.tel);
    }
}

上面的代码需要 ASM 工具才能进行编译，ASM 可以到 http://asm.objectweb.org/ 上面去下载。

执行顺序：

javac Student.java
javac -cp .;lib/asm-all-3.1.jar PreCompileProcess.java
java -cp .;lib/asm-all-3.1.jar PreCompileProcess
javac Test.java
java Test

在 Student.java 没有修改的情况下，PreCompileProcess 只需要执行一次就可以了。