javolution37.javolution.io

Class Struct

java.lang.Object

javolution37.javolution.io.Struct

Direct Known Subclasses:

Union

public class Struct
extends java.lang.Object

This class represents a C/C++ struct; it confers interoperability between Java classes and C/C++ struct.

Unlike C/C++, the storage layout of Java objects is not determined by the compiler. The layout of objects in memory is deferred to run time and determined by the interpreter (or just-in-time compiler). This approach allows for dynamic loading and binding; but also makes interfacing with C/C++ code difficult. Hence, this class for which the memory layout is defined by the initialization order of the Struct's members and follows the same alignment rules as C/C++ structs.

This class (as well as the Union sub-class) facilitates:

• Memory sharing between Java applications and native libraries.
• Direct encoding/decoding of streams for which the structure is defined by legacy C/C++ code.
• Serialization/deserialization of Java objects (complete control, e.g. no class header)
• Mapping of Java objects to physical addresses (with JNI).

Because of its one-to-one mapping, it is relatively easy to convert C header files (e.g. OpenGL bindings) to Java Struct/Union using simple text macros. Here is an example of C struct:

     struct Date {
         unsigned short year;
         unsigned byte month;
         unsigned byte day;
     };
     struct Student {
         char        name[64];
         struct Date birth;
         float       grades[10];
         Student*    next;
     };

and here is the Java equivalent using this class:

     public static class Date extends Struct {
         public final Unsigned16 year = new Unsigned16();
         public final Unsigned8 month = new Unsigned8();
         public final Unsigned8 day   = new Unsigned8();
     }
     public static class Student extends Struct {
         public final Utf8String  name   = new Utf8String(64);
         public final Date        birth  = inner(new Date());
         public final Float32[]   grades = array(new Float32[10]);
         public final Reference32<Student> next =  new Reference32<Student>();
     }

Struct's members are directly accessible:

     Student student = new Student();
     student.name.set("John Doe"); // Null terminated (C compatible)
     int age = 2003 - student.birth.year.get();
     student.grades[2].set(12.5f);
     student = student.next.get();

Applications may also work with the raw bytes directly. The following illustrate how Struct can be used to decode/encode UDP messages directly:

     class UdpMessage extends Struct {
          Unsigned16 xxx = new Unsigned16();
          ... 
     }
     public void run() {
         byte[] bytes = new byte[1024];
         DatagramPacket packet = new DatagramPacket(bytes, bytes.length);
         UdpMessage message = new UdpMessage();
         message.setByteBuffer(ByteBuffer.wrap(bytes), 0);
         // packet and message are now two different views of the same data. 
         while (isListening) {
             multicastSocket.receive(packet);
             int xxx = message.xxx.get();
             ... // Process message fields directly.
         }
     }

It is relatively easy to map instances of this class to any physical address using JNI. Here is an example:

     import java.nio.ByteBuffer;
     class Clock extends Struct { // Hardware clock mapped to memory.
         Unsigned16 seconds  = new Unsigned16(5); // unsigned short seconds:5
         Unsigned16 minutes  = new Unsigned16(5); // unsigned short minutes:5
         Unsigned16 hours    = new Unsigned16(4); // unsigned short hours:4
         Clock() {
             setByteBuffer(Clock.nativeBuffer(), 0);
         }
         private static native ByteBuffer nativeBuffer();
     }

Below is the nativeBuffer() implementation (Clock.c):

     #include 
     #include "Clock.h" // Generated using javah
     JNIEXPORT jobject JNICALL Java_Clock_nativeBuffer (JNIEnv *env, jclass) {
         return (*env)->NewDirectByteBuffer(env, clock_address, buffer_size)
     }

Bit-fields are supported (see Clock example above). Bit-fields allocation order is defined by the Struct byteOrder() return value (leftmost bit to rightmost bit if BIG_ENDIAN and rightmost bit to leftmost bit if LITTLE_ENDIAN). Unless the Struct packing directive is overriden, bit-fields cannot straddle the storage-unit boundary as defined by their base type (padding is inserted at the end of the first bit-field and the second bit-field is put into the next storage unit).

Finally, it is possible to change the ByteBuffer and/or the Struct position in its ByteBuffer to allow for a single Struct object to encode/decode multiple memory mapped instances.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
class	Struct.Bool This class represents a 8 bits boolean with true represented by 1 and false represented by 0.
class	Struct.Enum16 This class represents a 16 bits Enum.
class	Struct.Enum32 This class represents a 32 bits Enum.
class	Struct.Enum64 This class represents a 64 bits Enum.
class	Struct.Enum8 This class represents a 8 bits Enum.
class	Struct.Float32 This class represents a 32 bits float (C/C++/Java float).
class	Struct.Float64 This class represents a 64 bits float (C/C++/Java double).
class	Struct.Reference32<S extends Struct> This class represents a 32 bits reference (C/C++ pointer) to a Struct object (other types may require a Struct wrapper).
class	Struct.Reference64<S extends Struct> This class represents a 64 bits reference (C/C++ pointer) to a Struct object (other types may require a Struct wrapper).
class	Struct.Signed16 This class represents a 16 bits signed integer.
class	Struct.Signed32 This class represents a 32 bits signed integer.
class	Struct.Signed64 This class represents a 64 bits signed integer.
class	Struct.Signed8 This class represents a 8 bits signed integer.
class	Struct.Unsigned16 This class represents a 16 bits unsigned integer.
class	Struct.Unsigned32 This class represents a 32 bits unsigned integer.
class	Struct.Unsigned8 This class represents a 8 bits unsigned integer.
class	Struct.Utf8String This class represents a UTF-8 character string, null terminated (for C/C++ compatibility)

Constructor Summary

Constructors

Constructor and Description
Struct() Default constructor.

Method Summary

Modifier and Type	Method and Description
long	address() Returns this struct address.
java.nio.ByteOrder	byteOrder() Returns the byte order for this struct (configurable).
java.nio.ByteBuffer	getByteBuffer() Returns the byte buffer for this struct.
int	getByteBufferPosition() Returns the absolute position of this struct within its associated byte buffer.
boolean	isPacked() Indicates if this struct is packed (configurable).
boolean	isUnion() Indicates if this struct's members are mapped to the same location in memory (default false).
int	read(java.io.InputStream in) Reads this struct from the specified input stream (convenience method when using Stream I/O).
Struct	setByteBuffer(java.nio.ByteBuffer byteBuffer, int position) Sets the current byte buffer for this struct.
Struct	setByteBufferPosition(int position) Sets the position of this struct within its byte buffer.
int	size() Returns the size in bytes of this struct.
java.lang.String	toString() Returns the String representation of this struct in the form of its constituing bytes (hexadecimal).
void	write(java.io.OutputStream out) Writes this struct to the specified output stream (convenience method when using Stream I/O).

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

Struct

public Struct()

Default constructor.

Method Detail

size

public final int size()

Returns the size in bytes of this struct. The size includes tail padding to satisfy the struct alignment requirement (defined by the largest alignment of its members).

Returns:

the C/C++ sizeof(this).

getByteBuffer

public final java.nio.ByteBuffer getByteBuffer()

Returns the byte buffer for this struct. This method will allocate a new direct buffer if none has been set.

Changes to the buffer's content are visible in this struct, and vice versa.

The buffer of an inner struct is the same as its parent struct.

The position of a struct's member within the byte buffer is given by member.position()

Returns:

the current byte buffer or a new direct buffer if none set.

See Also:

setByteBuffer(java.nio.ByteBuffer, int)

setByteBuffer

public final Struct setByteBuffer(java.nio.ByteBuffer byteBuffer,
                                  int position)

Sets the current byte buffer for this struct. The specified byte buffer can be mapped to memory for direct memory access or can wrap a shared byte array for I/O purpose (e.g. DatagramPacket).

Parameters:

byteBuffer - the new byte buffer.

position - the position of this struct in the specified byte buffer.

Returns:

this

Throws:

java.lang.IllegalArgumentException - if the specified byteBuffer has a different byte order than this struct.

java.lang.UnsupportedOperationException - if this struct is an inner struct.

See Also:

byteOrder()

setByteBufferPosition

public final Struct setByteBufferPosition(int position)

Sets the position of this struct within its byte buffer.

Parameters:

position - the position of this struct in its byte buffer.

Returns:

this

Throws:

java.lang.UnsupportedOperationException - if this struct is an inner struct.

getByteBufferPosition

public final int getByteBufferPosition()

Returns the absolute position of this struct within its associated byte buffer.

Returns:

the absolute position of this struct in the byte buffer.

read

public int read(java.io.InputStream in)
         throws java.io.IOException

Reads this struct from the specified input stream (convenience method when using Stream I/O). For better performance, use of Block I/O (e.g. java.nio.channels.*) is recommended.

Parameters:

in - the input stream being read from.

Returns:

the number of bytes read (typically the size of this struct.

Throws:

java.io.IOException - if an I/O error occurs.

write

public void write(java.io.OutputStream out)
           throws java.io.IOException

Writes this struct to the specified output stream (convenience method when using Stream I/O). For better performance, use of Block I/O (e.g. java.nio.channels.*) is recommended.

Parameters:

out - the output stream to write to.

Throws:

java.io.IOException - if an I/O error occurs.

address

public final long address()

Returns this struct address. This method allows for structs to be referenced (e.g. pointer) from other structs.

Returns:

the struct memory address.

Throws:

java.lang.UnsupportedOperationException - if the struct buffer is not a direct buffer.

See Also:

Struct.Reference32, Struct.Reference64

toString

public java.lang.String toString()

Returns the String representation of this struct in the form of its constituing bytes (hexadecimal). For example:

     public static class Student extends Struct {
         Utf8String name  = new Utf8String(16);
         Unsigned16 year  = new Unsigned16();
         Float32    grade = new Float32();
     }
     Student student = new Student();
     student.name.set("John Doe");
     student.year.set(2003);
     student.grade.set(12.5f);
     System.out.println(student);

     4A 6F 68 6E 20 44 6F 65 00 00 00 00 00 00 00 00
     07 D3 00 00 41 48 00 00

Overrides:

toString in class java.lang.Object

Returns:

a hexadecimal representation of the bytes content for this struct.

isUnion

public boolean isUnion()

Indicates if this struct's members are mapped to the same location in memory (default false). This method is useful for applications extending Struct with new member types in order to create unions from these new structs. For example:

 public abstract class FortranStruct extends Struct {
     public class FortranString extends Member {...}
     protected FortranString[] array(FortranString[] array, int stringLength) { ... }
 }
 public abstract class FortranUnion extends FortranStruct {
     // Inherits new members and methods.
     public final isUnion() {
         return true;
     }
 }

Returns:

true if this struct's members are mapped to to the same location in memory; false otherwise.

See Also:

Union

byteOrder

public java.nio.ByteOrder byteOrder()

Returns the byte order for this struct (configurable). The byte order is inherited by inner structs. Sub-classes may change the byte order by overriding this method. For example:

 public class TopStruct extends Struct {
     ... // Members initialization.
     public ByteOrder byteOrder() {
         // TopStruct and its inner structs use hardware byte order.
         return ByteOrder.nativeOrder();
    }
 }}

Returns:

the byte order when reading/writing multibyte values (default: network byte order, BIG_ENDIAN).

isPacked

public boolean isPacked()

Indicates if this struct is packed (configurable). By default, members of a struct are aligned on the boundary corresponding to the member base type; padding is performed if necessary. This directive is inherited by inner structs. Sub-classes may change the packing directive by overriding this method. For example:

 public class TopStruct extends Struct {
     ... // Members initialization.
     public boolean isPacked() {
         // TopStruct and its inner structs are packed.
         return true;
     }
 }}

Returns:

true if alignment requirements are ignored. false otherwise (default).