Question

As part of an exercise I had to write a remote class loader. It downloads a class from a server and then loads it using defineClass.

What I did was something like this:

in=new ObjectInputStream(s.getInputStream());
Object o = in.readObject();
return defineClass(className,o.toString().getBytes(), 0, o.toString().length());

And I got a java.lang.ClassFormatError: Incompatible magic value

But when I wrote this

in=new ObjectInputStream(s.getInputStream());
byte[] classData=(byte[])in.readObject();
return defineClass(className,classData, 0, classData.length);

It worked as expected.

Why is that?

Answer 1

Object.toString() string returns you human readable form of Object So if you call toString on array object then it will return you default toString.

Default toString implementation is

public String toString() {
return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

You can clearly see this is not what you wanted.

You can use String constructor with Charset or default charset to convert byte[]to string

byte[] utf8Bytes = original.getBytes("UTF8");
String roundTrip = new String(utf8Bytes, "UTF8");

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References:

• Byte Encodings and Strings
• String JavaDoc