java - 如何避免运行静态初始化程序？

Question

我有一个遗留的(不可修改的 :-() 代码，它有一个静态初始化 block 。 我想创建一个子类 - 但不运行静态 block 。

有什么办法吗？例如:class.ForName 方法有第二个参数，它负责初始化类 - 但遗憾的是，它对我不起作用(可能意味着别的......):http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#forName(java.lang.String , boolean 值, java.lang.ClassLoader)

更新:上面提到的 class.forName 不会触发初始化 - 但请求 newInstance 会:-(

谢谢， 克丽丝

Answer 1

您始终可以使用 ASM 修补旧类.通过忽略 clinit block 从旧字节码生成新类应该很容易。

import org.objectweb.asm.*;
import org.objectweb.asm.commons.EmptyVisitor;
import java.io.*;

public class ClinitKiller {
    public static void main (String[] args) {
        final InputStream input = ClinitKiller.class.getResourceAsStream(Test.class.getName() + ".class");
        try {
            final byte[] bytes = instrument(input);
            FileOutputStream out = new FileOutputStream("/tmp/Test.class");
            out.write(bytes);
            out.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static byte[] instrument(InputStream is) throws IOException {
        ClassReader reader = new ClassReader(is);
        ClassWriter writer = new ClassWriter(0);
        ClinitKillerClassAdapter classAdapter = new ClinitKillerClassAdapter(writer);
        reader.accept(classAdapter, 0);
        return writer.toByteArray();
    }
}

class ClinitKillerClassAdapter extends ClassAdapter {
    public ClinitKillerClassAdapter(final ClassVisitor cv) {
        super(cv);
    }

    public MethodVisitor visitMethod(final int access, final String name, final String desc, final String signature, final String[] exceptions) {
        if (name.equals("<clinit>")) {
            return new EmptyVisitor();
        }
        return cv.visitMethod(access, name, desc, signature, exceptions);
    }
}

这是以下类(class)的前后对比:

public class Test {
    private static final String value;
    static {
        System.out.println("Test static");
        value = "test value";
    }
    public static void main(String[] args) {
        System.out.println(value);
    }
}

之前:

javap -c Test
Compiled from "Test.java"
public class Test extends java.lang.Object{
    public Test();
    Code:
    0:  aload_0
    1:  invokespecial   #1; //Method java/lang/Object."<init>":()V
    4:  return

    public static void main(java.lang.String[]);
    Code:
    0:  getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
    3:  getstatic   #3; //Field value:Ljava/lang/String;
    6:  invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
    9:  return

    static {};
    Code:
    0:  getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
    3:  ldc #5; //String Test static
    5:  invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
    8:  ldc #6; //String test value
    10: putstatic   #3; //Field value:Ljava/lang/String;
    13: return

}

输出:
测试静态
测试值

之后:

javap -c Test
Compiled from "Test.java"
public class Test extends java.lang.Object{
    public Test();
    Code:
    0:  aload_0
    1:  invokespecial   #11; //Method java/lang/Object."<init>":()V
    4:  return

    public static void main(java.lang.String[]);
    Code:
    0:  getstatic   #21; //Field java/lang/System.out:Ljava/io/PrintStream;
    3:  getstatic   #23; //Field value:Ljava/lang/String;
    6:  invokevirtual   #29; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
    9:  return

}

输出:
空