我使用 Java ASM 编写了一个简单的程序来生成通过编译以下类生成的字节码。
public class Main {
public static void main(String[] args) {
System.out.println("Test");
}
}
下面给出了我编写的为此类生成字节码的代码。
public class CodeGenerator {
public void generateClass()
{
ClassWriter cw=new ClassWriter(Opcodes.NULL);
FieldVisitor fv;
MethodVisitor mv;
cw.visit(Opcodes.V1_1, Opcodes.ACC_PUBLIC, "Main", null, "java/lang/Object", null);
mv=cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null);
mv.visitVarInsn(Opcodes.AALOAD, 0);
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
mv.visitInsn(Opcodes.RETURN);
mv.visitMaxs(1, 1);
mv.visitEnd();
mv=cw.visitMethod(Opcodes.ACC_PUBLIC+ Opcodes.ACC_STATIC, "Main", "([Ljava/lang/String;)V", null, null);
mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream");
mv.visitLdcInsn("Test");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V");
mv.visitInsn(Opcodes.RETURN);
mv.visitMaxs(2, 1);
mv.visitEnd();
cw.visitEnd();
this.WriteClass(cw);
}
public void WriteClass(ClassWriter cw){
FileOutputStream fos;
try{
fos = new FileOutputStream("E:\\Acadamic\\Final year project\\ASM\\Main.class");
fos.write(cw.toByteArray());
fos.close();
}
catch (IOException ex){
Logger.getLogger(CodeGenerator.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
编译成功,当我尝试运行它时,出现以下错误。
Exception in thread "main" java.lang.NoClassDefFoundError: main/class
Caused by: java.lang.ClassNotFoundException: main.class
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: main.class. Program will exit.
任何人都可以帮助我避免此错误。如果有人可以解释错误的原因以及如何避免它,请善意地帮助我。
谢谢。
后续:
当我像 java Main 一样运行它时,会出现以下错误。
Exception in thread "main" java.lang.ClassFormatError: Field "out" in class Main
has illegal signature "Ljava/io/PrintStream"
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: Main. Program will exit.
最佳答案
我认为您的错误不是由 ASM 引起的。看来您只是错误地调用了 java
。你应该这样调用它:
java Main
My guess is that you called it like this:
java Main.class
Update
There are three problems with your code.
Problem #1
The ClassWriter
constructor accepts a bit mask of options. You passed it Opcodes.NULL
. Although this may seem correct, you actually want to pass it 0
(Opcodes.NULL
means something different).
ClassWriter cw=new ClassWriter(0);
问题#2
您正在构造函数中调用 AALOAD
,而不是 ALOAD
。 AALOAD
指令加载数组元素,而ALOAD
加载局部变量。
mv.visitVarInsn(Opcodes.ALOAD, 0);
问题#3
在您的Main
方法中,您忘记了Ljava/io/PrintStream
后面的分号:
mv.visitFieldInsn(
Opcodes.GETSTATIC,
"java/lang/System",
"out",
"Ljava/io/PrintStream;" // Notice the semicolon
);
您可以通过利用 ASM 的 Type
类来减轻自己的痛苦:
mv.visitFieldInsn(
Opcodes.GETSTATIC,
"java/lang/System",
"out",
Type.getObjectType("java/io/PrintStream").getDescriptor()
);
关于Java ASM 需要帮助,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/4853767/