Byte buffers in java

Byte buffers in java

Byte buffers can be created either by allocation , which allocates space for the buffer’s content, or by wrapping an existing byte array into a buffer.

Direct vs. non-direct buffers

A byte buffer is either direct or non-direct. Given a direct byte buffer, the Java virtual machine will make a best effort to perform native I/O operations directly upon it. That is, it will attempt to avoid copying the buffer’s content to (or from) an intermediate buffer before (or after) each invocation of one of the underlying operating system’s native I/O operations.

A direct byte buffer may be created by invoking the allocateDirect factory method of this class. The buffers returned by this method typically have somewhat higher allocation and deallocation costs than non-direct buffers. The contents of direct buffers may reside outside of the normal garbage-collected heap, and so their impact upon the memory footprint of an application might not be obvious. It is therefore recommended that direct buffers be allocated primarily for large, long-lived buffers that are subject to the underlying system’s native I/O operations. In general it is best to allocate direct buffers only when they yield a measureable gain in program performance.

A direct byte buffer may also be created by mapping a region of a file directly into memory. An implementation of the Java platform may optionally support the creation of direct byte buffers from native code via JNI. If an instance of one of these kinds of buffers refers to an inaccessible region of memory then an attempt to access that region will not change the buffer’s content and will cause an unspecified exception to be thrown either at the time of the access or at some later time.

Whether a byte buffer is direct or non-direct may be determined by invoking its isDirect method. This method is provided so that explicit buffer management can be done in performance-critical code.

Access to binary data

This class defines methods for reading and writing values of all other primitive types, except boolean. Primitive values are translated to (or from) sequences of bytes according to the buffer’s current byte order, which may be retrieved and modified via the order methods. Specific byte orders are represented by instances of the ByteOrder class. The initial order of a byte buffer is always BIG_ENDIAN .

For access to heterogeneous binary data, that is, sequences of values of different types, this class defines a family of absolute and relative get and put methods for each type. For 32-bit floating-point values, for example, this class defines:

float getFloat() float getFloat(int index) void putFloat(float f) void putFloat(int index, float f)

Corresponding methods are defined for the types char, short, int, long, and double. The index parameters of the absolute get and put methods are in terms of bytes rather than of the type being read or written.

For access to homogeneous binary data, that is, sequences of values of the same type, this class defines methods that can create views of a given byte buffer. A view buffer is simply another buffer whose content is backed by the byte buffer. Changes to the byte buffer’s content will be visible in the view buffer, and vice versa; the two buffers’ position, limit, and mark values are independent. The asFloatBuffer method, for example, creates an instance of the FloatBuffer class that is backed by the byte buffer upon which the method is invoked. Corresponding view-creation methods are defined for the types char, short, int, long, and double.

• A view buffer is indexed not in terms of bytes but rather in terms of the type-specific size of its values;
• A view buffer provides relative bulk get and put methods that can transfer contiguous sequences of values between a buffer and an array or some other buffer of the same type; and
• A view buffer is potentially much more efficient because it will be direct if, and only if, its backing byte buffer is direct.

The byte order of a view buffer is fixed to be that of its byte buffer at the time that the view is created.

Invocation chaining

Methods in this class that do not otherwise have a value to return are specified to return the buffer upon which they are invoked. This allows method invocations to be chained. The sequence of statements

bb.putInt(0xCAFEBABE); bb.putShort(3); bb.putShort(45);

bb.putInt(0xCAFEBABE).putShort(3).putShort(45);

Method Summary

Returns the offset within this buffer’s backing array of the first element of the buffer (optional operation).

Methods inherited from class java.nio.Buffer

Methods inherited from class java.lang.Object

Method Detail

allocateDirect

public static ByteBuffer allocateDirect(int capacity)

Allocates a new direct byte buffer. The new buffer’s position will be zero, its limit will be its capacity, its mark will be undefined, and each of its elements will be initialized to zero. Whether or not it has a backing array is unspecified.

allocate

Allocates a new byte buffer. The new buffer’s position will be zero, its limit will be its capacity, its mark will be undefined, and each of its elements will be initialized to zero. It will have a backing array , and its array offset will be zero.

wrap

public static ByteBuffer wrap(byte[] array, int offset, int length)

Wraps a byte array into a buffer. The new buffer will be backed by the given byte array; that is, modifications to the buffer will cause the array to be modified and vice versa. The new buffer’s capacity will be array.length, its position will be offset, its limit will be offset + length, and its mark will be undefined. Its backing array will be the given array, and its array offset will be zero.

wrap

Wraps a byte array into a buffer. The new buffer will be backed by the given byte array; that is, modifications to the buffer will cause the array to be modified and vice versa. The new buffer’s capacity and limit will be array.length, its position will be zero, and its mark will be undefined. Its backing array will be the given array, and its array offset> will be zero.

slice

Creates a new byte buffer whose content is a shared subsequence of this buffer’s content. The content of the new buffer will start at this buffer’s current position. Changes to this buffer’s content will be visible in the new buffer, and vice versa; the two buffers’ position, limit, and mark values will be independent. The new buffer’s position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer, and its mark will be undefined. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

duplicate

Creates a new byte buffer that shares this buffer’s content. The content of the new buffer will be that of this buffer. Changes to this buffer’s content will be visible in the new buffer, and vice versa; the two buffers’ position, limit, and mark values will be independent. The new buffer’s capacity, limit, position, and mark values will be identical to those of this buffer. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

asReadOnlyBuffer

Creates a new, read-only byte buffer that shares this buffer’s content. The content of the new buffer will be that of this buffer. Changes to this buffer’s content will be visible in the new buffer; the new buffer itself, however, will be read-only and will not allow the shared content to be modified. The two buffers’ position, limit, and mark values will be independent. The new buffer’s capacity, limit, position, and mark values will be identical to those of this buffer. If this buffer is itself read-only then this method behaves in exactly the same way as the duplicate method.

get

Relative get method. Reads the byte at this buffer’s current position, and then increments the position.

put

Relative put method (optional operation). Writes the given byte into this buffer at the current position, and then increments the position.

get

public abstract byte get(int index)

put

Absolute put method (optional operation). Writes the given byte into this buffer at the given index.

get

public ByteBuffer get(byte[] dst, int offset, int length)

Relative bulk get method. This method transfers bytes from this buffer into the given destination array. If there are fewer bytes remaining in the buffer than are required to satisfy the request, that is, if length > remaining(), then no bytes are transferred and a BufferUnderflowException is thrown. Otherwise, this method copies length bytes from this buffer into the given array, starting at the current position of this buffer and at the given offset in the array. The position of this buffer is then incremented by length. In other words, an invocation of this method of the form src.get(dst, off, len) has exactly the same effect as the loop

except that it first checks that there are sufficient bytes in this buffer and it is potentially much more efficient.

get

Relative bulk get method. This method transfers bytes from this buffer into the given destination array. An invocation of this method of the form src.get(a) behaves in exactly the same way as the invocation

put

Relative bulk put method (optional operation). This method transfers the bytes remaining in the given source buffer into this buffer. If there are more bytes remaining in the source buffer than in this buffer, that is, if src.remaining() > remaining(), then no bytes are transferred and a BufferOverflowException is thrown. Otherwise, this method copies n = src.remaining() bytes from the given buffer into this buffer, starting at each buffer’s current position. The positions of both buffers are then incremented by n. In other words, an invocation of this method of the form dst.put(src) has exactly the same effect as the loop

while (src.hasRemaining()) dst.put(src.get());

except that it first checks that there is sufficient space in this buffer and it is potentially much more efficient.

put

public ByteBuffer put(byte[] src, int offset, int length)

Relative bulk put method (optional operation). This method transfers bytes into this buffer from the given source array. If there are more bytes to be copied from the array than remain in this buffer, that is, if length > remaining(), then no bytes are transferred and a BufferOverflowException is thrown. Otherwise, this method copies length bytes from the given array into this buffer, starting at the given offset in the array and at the current position of this buffer. The position of this buffer is then incremented by length. In other words, an invocation of this method of the form dst.put(src, off, len) has exactly the same effect as the loop

except that it first checks that there is sufficient space in this buffer and it is potentially much more efficient.

put

Relative bulk put method (optional operation). This method transfers the entire content of the given source byte array into this buffer. An invocation of this method of the form dst.put(a) behaves in exactly the same way as the invocation

hasArray

public final boolean hasArray()

Tells whether or not this buffer is backed by an accessible byte array. If this method returns true then the array and arrayOffset methods may safely be invoked.

array

Returns the byte array that backs this buffer (optional operation). Modifications to this buffer’s content will cause the returned array’s content to be modified, and vice versa. Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.

arrayOffset

public final int arrayOffset()

Returns the offset within this buffer’s backing array of the first element of the buffer (optional operation). If this buffer is backed by an array then buffer position p corresponds to array index p + arrayOffset(). Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.

compact

Compacts this buffer (optional operation). The bytes between the buffer’s current position and its limit, if any, are copied to the beginning of the buffer. That is, the byte at index p = position() is copied to index zero, the byte at index p + 1 is copied to index one, and so forth until the byte at index limit() — 1 is copied to index n = limit() — 1 — p. The buffer’s position is then set to n+1 and its limit is set to its capacity. The mark, if defined, is discarded. The buffer’s position is set to the number of bytes copied, rather than to zero, so that an invocation of this method can be followed immediately by an invocation of another relative put method. Invoke this method after writing data from a buffer in case the write was incomplete. The following loop, for example, copies bytes from one channel to another via the buffer buf:

 buf.clear(); // Prepare buffer for use while (in.read(buf) >= 0 || buf.position != 0) < buf.flip(); out.write(buf); buf.compact(); // In case of partial write >

isDirect

public abstract boolean isDirect()

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