CODEWITHSUNDEEP
javastruct
krb
krb

Reputation: 16315

Is it possible to use struct-like constructs in Java?

I'm considering using Java for a large project but I haven't been able to find anything that remotely represented structures in Java. I need to be able to convert network packets to structures/classes that can be used in the application.

I know that it is possible to use RandomAccessFile but this way is NOT acceptable. So I'm curious if it is possible to "cast" a set of bytes to a structure like I could do in C. If this is not possible then I cannot use Java.

So the question I'm asking is if it is possible to cast aligned data to a class without any extra effort beyond specifying the alignment and data types?

Upvotes: 8

Views: 8826

Answers (8)

dacwe
dacwe

Reputation: 43504

No. You cannot cast a array of bytes to a class object.

That being said, you can use a java.nio.Buffer and easily extract the fields you need to an object like this:

class Packet {

    private final int type;
    private final float data1;
    private final short data2;

    public Packet(byte[] bytes) {
        ByteBuffer bb = ByteBuffer.wrap(bytes);
        bb.order(ByteOrder.BIG_ENDIAN); // or LITTLE_ENDIAN

        type = bb.getInt();
        data1 = bb.getFloat();
        data2 = bb.getShort();
    }
}

Upvotes: 17

AlexR
AlexR

Reputation: 115328

You cannot cast array of bytes to instance of class. But you can do much much more with java. Java has internal, very strong and very flexible mechanism of serialization. This is what you need. You can read and write object to/from stream.

If both sides are written in java, there are no problem at all. If one of sides is not java you can customeze your serialization. Start from reading javadoc of java.util.Serializable.

Upvotes: -1

Vlad
Vlad

Reputation: 10780

ByteBuffer.wrap(new byte[] {}).getDouble();

Upvotes: 1

Mike Baranczak
Mike Baranczak

Reputation: 8374

No, this is not possible. You're trying to use Java like C, which is bound to cause complications. Either learn to do things the Java way, or go back to C.

In this case, the Java way would probably involve DataInputStream and/or DataOutputStream.

Upvotes: 0

Ray Toal
Ray Toal

Reputation: 88378

As Joshua says, serialization is the typical way to do these kinds of things. However you there are other binary protocols like MessagePack, ProtocolBuffers, and AvRO.

If you want to play with the bytecode structures, look at ASM and CGLIB; these are very common in Java applications.

Upvotes: 2

Cameron Skinner
Cameron Skinner

Reputation: 54286

No, you cannot do that. Java simply doesn't have the same concepts as C.

You can create a class that behaves much like a struct:

public class Structure {
    public int field1;
    public String field2;
}

and you can have a constructor that takes an array or bytes or a DataInput to read the bytes:

public class Structure {
    ...
    public Structure(byte[] data) {
        this(new DataInputStream(new ByteArrayInputStream(data)));
    }

    public Structure(DataInput in) {
        field1 = in.readInt();
        field2 = in.readUTF();
    }
}

then read bytes off the wire and pump them into Structures:

byte[] bytes = network.read();
DataInputStream stream = new DataInputStream(new ByteArrayInputStream(bytes));
Structure structure1 = new Structure(stream);
Structure structure2 = new Structure(stream);
...

It's not as concise as C but it's pretty close. Note that the DataInput interface cleanly removes any mucking around with endianness on your behalf, so that's definitely a benefit over C.

Upvotes: 4

Confusion
Confusion

Reputation: 16841

You're basically asking whether you can use a C-specific solution to a problem in another language. The answer is, predictably, 'no'.

However, it is perfectly possible to construct a class that takes a set of bytes in its constructor and constructs an appropriate instance.

class Foo {

  int someField;
  String anotherField;

  public Foo(byte[] bytes) {
    someField = someFieldFromBytes(bytes);
    anotherField = anotherFieldFromBytes(bytes);
    etc.
 }
}

You can ensure there is a one-to-one mapping of class instances to byte arrays. Add a toBytes() method to serialize an instance into bytes.

Upvotes: 9

Joshua Kaplan
Joshua Kaplan

Reputation: 883

There is nothing which matches your description.

The closest thing to a struct in Java is a simple class which holds values either accessible through it's fields or set/get methods.

The typical means to convert between Java class instances and on-the-wire representations is Java serialization which can be heavily customized as need be. It is what is used by Java's Remote Method Invocation API and works extremely well.

Upvotes: 1

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