javolution.io

Class Struct

java.lang.Object

javolution.io.Struct

Direct Known Subclasses:

Union

@Realtime
public class Struct
extends Object

Equivalent to a C/C++ struct; this class 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 wordSize 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:

     enum Gender{MALE, FEMALE};
     struct Date {
         unsigned short year;
         unsigned byte month;
         unsigned byte day;
     };
     struct Student {
         enum Gender gender;
         char        name[64];
         struct Date birth;
         float       grades[10];
         Student*    next;
     };

and here is the Java equivalent using this class:

     public enum Gender { MALE, FEMALE };
     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 Enum32<Gender>       gender = new Enum32<Gender>(Gender.values());
         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.gender.set(Gender.MALE);
     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 can 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 <jni.h>
     #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 (same layout as Microsoft Visual C++). C/C++ 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). It is possible to avoid bit padding by using the Struct.BitField member (or a sub-class). In which case the allocation order is always from the leftmost to the rightmost bit (same as BIG_ENDIAN).

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.

Note: Because Struct/Union are basically wrappers around java.nio.ByteBuffer, tutorials/usages for the Java NIO package are directly applicable to Struct/Union.

Version:

5.5.1, April 1, 2010

Author:

Jean-Marie Dautelle

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
class	Struct.BitField This class represents an arbitrary size (unsigned) bit field with no word size constraint (they can straddle words boundaries).
class	Struct.Bool This class represents a 8 bits boolean with true represented by 1 and false represented by 0.
class	Struct.Enum16<T extends Enum<T>> This class represents a 16 bits Enum.
class	Struct.Enum32<T extends Enum<T>> This class represents a 32 bits Enum.
class	Struct.Enum64<T extends Enum<T>> This class represents a 64 bits Enum.
class	Struct.Enum8<T extends Enum<T>> 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).
protected class	Struct.Member This inner class represents the base class for all Struct members.
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)

Field Summary

Fields

Modifier and Type	Field and Description
static LocalContext.Parameter<Integer>	MAXIMUM_ALIGNMENT Configurable holding the maximum wordSize in bytes (default 4).

Constructor Summary

Constructors

Constructor and Description
Struct() Default constructor.

Method Summary

Methods

Modifier and Type	Method and Description
long	address() Returns this struct address.
protected <M extends Struct.Member> M[]	array(M[] arrayMember) Defines the specified array member.
protected <M extends Struct.Member> M[][]	array(M[][] arrayMember) Defines the specified two-dimensional array member.
protected <M extends Struct.Member> M[][][]	array(M[][][] arrayMember) Defines the specified three-dimensional array member.
protected <S extends Struct> S[]	array(S[] structs) Defines the specified array of structs as inner structs.
protected <S extends Struct> S[][]	array(S[][] structs) Defines the specified two-dimensional array of structs as inner structs.
protected <S extends Struct> S[][][]	array(S[][][] structs) Defines the specified three dimensional array of structs as inner structs.
protected Struct.UTF8String[]	array(Struct.UTF8String[] array, int stringLength) Defines the specified array of UTF-8 strings, all strings having the specified length (convenience method).
ByteOrder	byteOrder() Returns the byte order for this struct (configurable).
ByteBuffer	getByteBuffer() Returns the byte buffer for this struct.
int	getByteBufferPosition() Returns the absolute byte position of this struct within its associated byte buffer.
protected <S extends Struct> S	inner(S struct) Defines the specified struct as inner of this struct.
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).
Struct	outer() Returns the outer of this struct or null if this struct is not an inner struct.
int	read(InputStream in) Reads this struct from the specified input stream (convenience method when using Stream I/O).
long	readBits(int bitOffset, int bitSize) Reads the specified bits from this Struct as an long (signed) integer value.
Struct	setByteBuffer(ByteBuffer byteBuffer, int position) Sets the current byte buffer for this struct.
Struct	setByteBufferPosition(int position) Sets the byte position of this struct within its byte buffer.
int	size() Returns the size in bytes of this struct.
String	toString() Returns the String representation of this struct in the form of its constituing bytes (hexadecimal).
void	write(OutputStream out) Writes this struct to the specified output stream (convenience method when using Stream I/O).
void	writeBits(long value, int bitOffset, int bitSize) Writes the specified bits into this Struct.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

MAXIMUM_ALIGNMENT

public static final LocalContext.Parameter<Integer> MAXIMUM_ALIGNMENT

Configurable holding the maximum wordSize in bytes (default 4). Should be a value greater or equal to 1.

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 word size requirement (defined by the largest word size of its members).

Returns:

the C/C++ sizeof(this).

outer

public Struct outer()

Returns the outer of this struct or null if this struct is not an inner struct.

Returns:

the outer struct or null.

getByteBuffer

public final 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.

If no byte buffer has been set, a direct buffer is allocated with a capacity equals to this struct's size.

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(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). The capacity of the specified byte buffer should be at least the size of this struct plus the offset position.

Parameters:

byteBuffer - the new byte buffer.

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

Returns:

this

Throws:

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

UnsupportedOperationException - if this struct is an inner struct.

See Also:

byteOrder()

setByteBufferPosition

public final Struct setByteBufferPosition(int position)

Sets the byte position of this struct within its byte buffer.

Parameters:

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

Returns:

this

Throws:

UnsupportedOperationException - if this struct is an inner struct.

getByteBufferPosition

public final int getByteBufferPosition()

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

Returns:

the absolute position of this struct (can be an inner struct) in the byte buffer.

read

public int read(InputStream in)
         throws 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. This method behaves appropriately when not all of the data is available from the input stream. Incomplete data is extremely common when the input stream is associated with something like a TCP connection. The typical usage pattern in those scenarios is to repeatedly call read() until the entire message is received.

Parameters:

in - the input stream being read from.

Returns:

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

Throws:

IOException - if an I/O error occurs.

write

public void write(OutputStream out)
           throws 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:

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:

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

See Also:

Struct.Reference32, Struct.Reference64

toString

public 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 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 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 not inherited by inner structs. Sub-classes may change the packing directive by overriding this method. For example:

 public class MyStruct extends Struct {
     ... // Members initialization.
     public boolean isPacked() {
         return true; // MyStruct is packed.
     }
 }}

Returns:

true if word size requirements are ignored. false otherwise (default).

inner

protected <S extends Struct> S inner(S struct)

Defines the specified struct as inner of this struct.

Parameters:

struct - the inner struct.

Returns:

the specified struct.

Throws:

IllegalArgumentException - if the specified struct is already an inner struct.

array

protected <S extends Struct> S[] array(S[] structs)

Defines the specified array of structs as inner structs. The array is populated if necessary using the struct component default constructor (which must be public).

Parameters:

structs - the struct array.

Returns:

the specified struct array.

Throws:

IllegalArgumentException - if the specified array contains inner structs.

array

protected <S extends Struct> S[][] array(S[][] structs)

Defines the specified two-dimensional array of structs as inner structs. The array is populated if necessary using the struct component default constructor (which must be public).

Parameters:

structs - the two dimensional struct array.

Returns:

the specified struct array.

Throws:

IllegalArgumentException - if the specified array contains inner structs.

array

protected <S extends Struct> S[][][] array(S[][][] structs)

Defines the specified three dimensional array of structs as inner structs. The array is populated if necessary using the struct component default constructor (which must be public).

Parameters:

structs - the three dimensional struct array.

Returns:

the specified struct array.

Throws:

IllegalArgumentException - if the specified array contains inner structs.

array

protected <M extends Struct.Member> M[] array(M[] arrayMember)

Defines the specified array member. For predefined members, the array is populated when empty; custom members should use literal (populated) arrays.

Parameters:

arrayMember - the array member.

Returns:

the specified array member.

Throws:

UnsupportedOperationException - if the specified array is empty and the member type is unknown.

array

protected <M extends Struct.Member> M[][] array(M[][] arrayMember)

Defines the specified two-dimensional array member. For predefined members, the array is populated when empty; custom members should use literal (populated) arrays.

Parameters:

arrayMember - the two-dimensional array member.

Returns:

the specified array member.

Throws:

UnsupportedOperationException - if the specified array is empty and the member type is unknown.

array

protected <M extends Struct.Member> M[][][] array(M[][][] arrayMember)

Defines the specified three-dimensional array member. For predefined members, the array is populated when empty; custom members should use literal (populated) arrays.

Parameters:

arrayMember - the three-dimensional array member.

Returns:

the specified array member.

Throws:

UnsupportedOperationException - if the specified array is empty and the member type is unknown.

array

protected Struct.UTF8String[] array(Struct.UTF8String[] array,
                        int stringLength)

Defines the specified array of UTF-8 strings, all strings having the specified length (convenience method).

Parameters:

array - the string array.

stringLength - the length of the string elements.

Returns:

the specified string array.

readBits

public long readBits(int bitOffset,
            int bitSize)

Reads the specified bits from this Struct as an long (signed) integer value.

Parameters:

bitOffset - the bit start position in the Struct.

bitSize - the number of bits.

Returns:

the specified bits read as a signed long.

Throws:

IllegalArgumentException - if (bitOffset + bitSize - 1) / 8 >= this.size()

writeBits

public void writeBits(long value,
             int bitOffset,
             int bitSize)

Writes the specified bits into this Struct.

Parameters:

value - the bits value as a signed long.

bitOffset - the bit start position in the Struct.

bitSize - the number of bits.

Throws:

IllegalArgumentException - if (bitOffset + bitSize - 1) / 8 >= this.size()