Java nio buffer flip

Class ByteBuffer

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 measurable 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) ByteBuffer putFloat(float f) ByteBuffer 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);

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Java nio buffer flip

A container for data of a specific primitive type. A buffer is a linear, finite sequence of elements of a specific primitive type. Aside from its content, the essential properties of a buffer are its capacity, limit, and position:

A buffer’s capacity is the number of elements it contains. The capacity of a buffer is never negative and never changes. A buffer’s limit is the index of the first element that should not be read or written. A buffer’s limit is never negative and is never greater than its capacity. A buffer’s position is the index of the next element to be read or written. A buffer’s position is never negative and is never greater than its limit.

Transferring data

Relative operations read or write one or more elements starting at the current position and then increment the position by the number of elements transferred. If the requested transfer exceeds the limit then a relative get operation throws a BufferUnderflowException and a relative put operation throws a BufferOverflowException ; in either case, no data is transferred. Absolute operations take an explicit element index and do not affect the position. Absolute get and put operations throw an IndexOutOfBoundsException if the index argument exceeds the limit.

Data may also, of course, be transferred in to or out of a buffer by the I/O operations of an appropriate channel, which are always relative to the current position.

Marking and resetting

A buffer’s mark is the index to which its position will be reset when the reset method is invoked. The mark is not always defined, but when it is defined it is never negative and is never greater than the position. If the mark is defined then it is discarded when the position or the limit is adjusted to a value smaller than the mark. If the mark is not defined then invoking the reset method causes an InvalidMarkException to be thrown.

Invariants

A newly-created buffer always has a position of zero and a mark that is undefined. The initial limit may be zero, or it may be some other value that depends upon the type of the buffer and the manner in which it is constructed. Each element of a newly-allocated buffer is initialized to zero.

Clearing, flipping, and rewinding

• clear() makes a buffer ready for a new sequence of channel-read or relative put operations: It sets the limit to the capacity and the position to zero.
• flip() makes a buffer ready for a new sequence of channel-write or relative get operations: It sets the limit to the current position and then sets the position to zero.
• rewind() makes a buffer ready for re-reading the data that it already contains: It leaves the limit unchanged and sets the position to zero.

Read-only buffers

Every buffer is readable, but not every buffer is writable. The mutation methods of each buffer class are specified as optional operations that will throw a ReadOnlyBufferException when invoked upon a read-only buffer. A read-only buffer does not allow its content to be changed, but its mark, position, and limit values are mutable. Whether or not a buffer is read-only may be determined by invoking its isReadOnly method.

Thread safety

Buffers are not safe for use by multiple concurrent threads. If a buffer is to be used by more than one thread then access to the buffer should be controlled by appropriate synchronization.

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; for example, the sequence of statements

b.flip(); b.position(23); b.limit(42);

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.lang.Object

Method Detail

capacity

public final int capacity()

position

public final int position()

position

Sets this buffer’s position. If the mark is defined and larger than the new position then it is discarded.

limit

limit

Sets this buffer’s limit. If the position is larger than the new limit then it is set to the new limit. If the mark is defined and larger than the new limit then it is discarded.

mark

reset

Resets this buffer’s position to the previously-marked position. Invoking this method neither changes nor discards the mark’s value.

clear

Clears this buffer. The position is set to zero, the limit is set to the capacity, and the mark is discarded. Invoke this method before using a sequence of channel-read or put operations to fill this buffer. For example:

buf.clear(); // Prepare buffer for reading in.read(buf); // Read data

This method does not actually erase the data in the buffer, but it is named as if it did because it will most often be used in situations in which that might as well be the case.

flip

Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded. After a sequence of channel-read or put operations, invoke this method to prepare for a sequence of channel-write or relative get operations. For example:

buf.put(magic); // Prepend header in.read(buf); // Read data into rest of buffer buf.flip(); // Flip buffer out.write(buf); // Write header + data to channel

This method is often used in conjunction with the compact method when transferring data from one place to another.

rewind

Rewinds this buffer. The position is set to zero and the mark is discarded. Invoke this method before a sequence of channel-write or get operations, assuming that the limit has already been set appropriately. For example:

out.write(buf); // Write remaining data buf.rewind(); // Rewind buffer buf.get(array); // Copy data into array

remaining

public final int remaining()

hasRemaining

public final boolean hasRemaining()

isReadOnly

public abstract boolean isReadOnly()

hasArray

public abstract boolean hasArray()

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

array

Returns the array that backs this buffer (optional operation). This method is intended to allow array-backed buffers to be passed to native code more efficiently. Concrete subclasses provide more strongly-typed return values for this method. 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 abstract 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.

isDirect

public abstract boolean isDirect()

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