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Documentation updates for working with DataBuffers
Issue: SPR-17409
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‎src/docs/asciidoc/core/core-databuffer-codec.adoc‎

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[[databuffers]]
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= Data Buffers and Codecs
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The `DataBuffer` interface defines an abstraction over byte buffers.
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The main reason for introducing it (and not using the standard `java.nio.ByteBuffer` instead) is Netty.
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Netty does not use `ByteBuffer` but instead offers `ByteBuf` as an alternative.
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Spring's `DataBuffer` is a simple abstraction over `ByteBuf` that can also be used on non-Netty
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platforms (that is, Servlet 3.1+).
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Java NIO provides `ByteBuffer` but many libraries build their own byte buffer API on top,
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especially for network operations where reusing buffers and/or using direct buffers is
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beneficial for performance. For example Netty has the `ByteBuf` hierarchy, Undertow uses
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XNIO, Jetty uses pooled byte buffers with a callback to be released, and so on.
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The `spring-core` module provides a set of abstractions to work with various byte buffer
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APIs as follows:
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* <<databuffers-factory>> abstracts the creation of a data buffer.
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* <<databuffers-buffer>> represents a byte buffer, which may be
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<<databuffers-buffer-pooled,pooled>>.
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* <<databuffers-utils>> offers utility methods for data buffers.
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* <<Codecs>> decode or encode streams data buffer streams into higher level objects.
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[[databuffers-factory]]
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== `DataBufferFactory`
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The `DataBufferFactory` offers functionality to allocate new data buffers as well as to wrap
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existing data.
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The `allocateBuffer` methods allocate a new data buffer with a default or given capacity.
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Though `DataBuffer` implementations grow and shrink on demand, it is more efficient to give the
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capacity upfront, if known.
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The `wrap` methods decorate an existing `ByteBuffer` or byte array.
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Wrapping does not involve allocation. It decorates the given data with a `DataBuffer`
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implementation.
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`DataBufferFactory` is used to create data buffers in one of two ways:
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There are two implementation of `DataBufferFactory`: the `NettyDataBufferFactory`
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(for Netty platforms, such as Reactor Netty) and `DefaultDataBufferFactory`
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(for other platforms, such as Servlet 3.1+ servers).
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. Allocate a new data buffer, optionally specifying capacity upfront, if known, which is
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more efficient even though implementations of `DataBuffer` can grow and shrink on demand.
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. Wrap an existing `byte[]` or `java.nio.ByteBuffer`, which decorates the given data with
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a `DataBuffer` implementation and that does not involve allocation.
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Note that WebFlux applications do not create a `DataBufferFactory` directly but instead
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access it through the `ServerHttpResponse` or the `ClientHttpRequest` on the client side.
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The type of factory depends on the underlying client or server, e.g.
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`NettyDataBufferFactory` for Reactor Netty, `DefaultDataBufferFactory` for others.
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== The `DataBuffer` Interface
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The `DataBuffer` interface is similar to `ByteBuffer` but offers a number of advantages.
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Similar to Netty's `ByteBuf`, the `DataBuffer` abstraction offers independent read and write
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positions.
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This is different from the JDK's `ByteBuffer`, which exposes only one position for both reading and
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writing and a separate `flip()` operation to switch between the two I/O operations.
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In general, the following invariant holds for the read position, write position, and the capacity:
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[[databuffers-buffer]]
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== `DataBuffer`
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====
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[literal]
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[subs="verbatim,quotes"]
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--
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0 <= read position <= write position <= capacity
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--
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====
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The `DataBuffer` interface offers similar operations as `java.nio.ByteBuffer` but also
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brings a few additional benefits some of which are inspired by the Netty `ByteBuf`.
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Below is a partial list of benefits:
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When reading bytes from the `DataBuffer`, the read position is automatically updated in accordance with
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the amount of data read from the buffer.
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Similarly, when writing bytes to the `DataBuffer`, the write position is updated with the amount of
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data written to the buffer.
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Also, when writing data, the capacity of a `DataBuffer` is automatically expanded, in the same fashion as `StringBuilder`,
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`ArrayList`, and similar types.
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Besides the reading and writing functionality mentioned above, the `DataBuffer` also has methods to
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view a (slice of a) buffer as a `ByteBuffer`, an `InputStream`, or an `OutputStream`.
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Additionally, it offers methods to determine the index of a given byte.
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As mentioned earlier, there are two implementation of `DataBufferFactory`: the `NettyDataBufferFactory`
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(for Netty platforms, such as Reactor Netty) and
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`DefaultDataBufferFactory` (for other platforms, such as
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Servlet 3.1+ servers).
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=== `PooledDataBuffer`
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The `PooledDataBuffer` is an extension to `DataBuffer` that adds methods for reference counting.
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The `retain` method increases the reference count by one.
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The `release` method decreases the count by one and releases the buffer's memory when the count
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reaches 0.
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Both of these methods are related to reference counting, a mechanism that we explain <<databuffer-reference-counting,later>>.
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Note that `DataBufferUtils` offers useful utility methods for releasing and retaining pooled data
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buffers.
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These methods take a plain `DataBuffer` as a parameter but only call `retain` or `release` if the
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passed data buffer is an instance of `PooledDataBuffer`.
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[[databuffer-reference-counting]]
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==== Reference Counting
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Reference counting is not a common technique in Java. It is much more common in other programming
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languages, such as Object C and C++.
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In and of itself, reference counting is not complex. It basically involves tracking the number of
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references that apply to an object.
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The reference count of a `PooledDataBuffer` starts at 1, is incremented by calling `retain`,
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and is decremented by calling `release`.
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As long as the buffer's reference count is larger than 0, the buffer is not released.
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When the number decreases to 0, the instance is released.
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In practice, this means that the reserved memory captured by the buffer is returned back to
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the memory pool, ready to be used for future allocations.
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In general, the last component to access a `DataBuffer` is responsible for releasing it.
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Within Spring, there are two sorts of components that release buffers: decoders and transports.
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Decoders are responsible for transforming a stream of buffers into other types (see <<codecs>>),
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and transports are responsible for sending buffers across a network boundary, typically as an HTTP message.
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This means that, if you allocate data buffers for the purpose of putting them into an outbound HTTP
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message (that is, a client-side request or server-side response), they do not have to be released.
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The other consequence of this rule is that if you allocate data buffers that do not end up in the
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body (for instance, because of a thrown exception), you have to release them yourself.
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The following snippet shows a typical `DataBuffer` usage scenario when dealing with methods that
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throw exceptions:
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* Read and write with independent positions, i.e. not requiring a call to `flip()` to
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alternate between read and write.
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* Capacity expanded on demand as with `java.lang.StringBuilder`.
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* Pooled buffers and reference counting via <<databuffers-buffer-pooled>>.
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* View a buffer as `java.nio.ByteBuffer`, `InputStream`, or `OutputStream`.
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* Determine the index, or the last index, for a given byte.
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====
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[source,java,indent=0]
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[subs="verbatim,quotes"]
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----
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DataBufferFactory factory = ...
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DataBuffer buffer = factory.allocateBuffer(); <1>
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boolean release = true; <2>
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try {
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writeDataToBuffer(buffer); <3>
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putBufferInHttpBody(buffer);
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release = false; <4>
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}
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finally {
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if (release) {
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DataBufferUtils.release(buffer); <5>
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}
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}
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private void writeDataToBuffer(DataBuffer buffer) throws IOException { <3>
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...
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}
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----
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<1> A new buffer is allocated.
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<2> A boolean flag indicates whether the allocated buffer should be released.
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<3> This example method loads data into the buffer. Note that the method can throw an `IOException`.
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Therefore, a `finally` block to release the buffer is required.
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<4> If no exception occurred, we switch the `release` flag to `false` as the buffer is now
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released as part of sending the HTTP body across the wire.
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<5> If an exception did occur, the flag is still set to `true`, and the buffer is released
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here.
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====
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[[databuffers-buffer-pooled]]
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== `PooledDataBuffer`
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As explained in the Javadoc for
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https://docs.oracle.com/javase/8/docs/api/java/nio/ByteBuffer.html[ByteBuffer],
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byte buffers can be direct or non-direct. Direct buffers may reside outside the Java heap
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which eliminates the need for copying for native I/O operations. That makes direct buffers
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particularly useful for receiving and sending data over a socket, but they're also more
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expensive to create and release, which leads to the idea of pooling buffers.
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`PooledDataBuffer` is an extension of `DataBuffer` that helps with reference counting which
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is essential for byte buffer pooling. How does it work? When a `PooledDataBuffer` is
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allocated the reference count is at 1. Calls to `retain()` increment the count, while
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calls to `release()` decrement it. As long as the count is above 0, the buffer is
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guaranteed not to be released. When the count is decreased to 0, the pooled buffer can be
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released, which in practice could mean the reserved memory for the buffer is returned to
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the memory pool.
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Note that instead of operating on `PooledDataBuffer` directly, in most cases it's better
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to use the convenience methods in `DataBufferUtils` that apply release or retain to a
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`DataBuffer` only if it is an instance of `PooledDataBuffer`.
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=== `DataBufferUtils`
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The `DataBufferUtils` class contains various utility methods that operate on data buffers.
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It contains methods for reading a `Flux` of `DataBuffer` objects from an `InputStream` or NIO
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`Channel` and methods for writing a data buffer `Flux` to an `OutputStream` or `Channel`.
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`DataBufferUtils` also exposes `retain` and `release` methods that operate on plain `DataBuffer`
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instances (so that casting to a `PooledDataBuffer` is not required).
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Additionally, `DataBufferUtils` exposes `compose`, which merges a stream of data buffers into one.
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For instance, this method can be used to convert the entire HTTP body into a single buffer (and
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from that, a `String` or `InputStream`).
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This is particularly useful when dealing with older, blocking APIs.
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Note, however, that this puts the entire body in memory, and therefore uses more memory than a pure
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streaming solution would.
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[[databuffers-utils]]
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== `DataBufferUtils`
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`DataBufferUtils` offers a number of utility methods to operate on data buffers:
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* Join a stream of data buffers into a single buffer possibly with zero copy, e.g. via
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composite buffers, if that's supported by the underlying byte buffer API.
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* Turn `InputStream` or NIO `Channel` into `Flux<DataBuffer>`, and vice versa a
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`Publisher<DataBuffer>` into `OutputStream` or NIO `Channel`.
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* Methods to release or retain a `DataBuffer` if the buffer is an instance of
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`PooledDataBuffer`.
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* Skip or take from a stream of bytes until a specific byte count.
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[[codecs]]
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== Codecs
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The `org.springframework.core.codec` package contains the two main abstractions for converting a
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stream of bytes into a stream of objects or vice-versa.
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The `Encoder` is a strategy interface that encodes a stream of objects into an output stream of
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data buffers.
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The `Decoder` does the reverse: It turns a stream of data buffers into a stream of objects.
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Note that a decoder instance needs to consider <<databuffer-reference-counting,reference counting>>.
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Spring comes with a wide array of default codecs (to convert from and to `String`,
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`ByteBuffer`, and byte arrays) and codecs that support marshalling libraries such as JAXB and
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Jackson (with https://github.com/FasterXML/jackson-core/issues/57[Jackson 2.9+ support for non-blocking parsing]).
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Within the context of Spring WebFlux, codecs are used to convert the request body into a
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`@RequestMapping` parameter or to convert the return type into the response body that is sent back
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to the client.
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The default codecs are configured in the `WebFluxConfigurationSupport` class. You can
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change them by overriding the `configureHttpMessageCodecs` when you inherit from that class.
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For more information about using codecs in WebFlux, see <<web-reactive#webflux-codecs>>.
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The `org.springframework.core.codec` package provides the following stragy interfaces:
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* `Encoder` to encode `Publisher<T>` into a stream of data buffers.
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* `Decoder` to decode `Publisher<DataBuffer>` into a stream of higher level objects.
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The `spring-core` module provides `byte[]`, `ByteBuffer`, `DataBuffer`, `Resource`, and
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`String` encoder and decoder implementations. The `spring-web` module adds Jackson JSON,
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Jackson Smile, JAXB2, Protocol Buffers and other encoders and decoders. See
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<<web-reactive.adoc#webflux-codecs,Codecs>> in the WebFlux section.
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[[databuffers-using]]
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== Using `DataBuffer`
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When working with data buffers, special care must be taken to ensure buffers are released
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since they may be <<databuffers-buffer-pooled,pooled>>. We'll use codecs to illustrate
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how that works but the concepts apply more generally. Let's see what codecs must do
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internally to manage data buffers.
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A `Decoder` is the last to read input data buffers, before creating higher level
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objects, and therefore it must release them as follows:
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. If a `Decoder` simply reads each input buffer and is ready to
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release it immediately, it can do so via `DataBufferUtils.release(dataBuffer)`.
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. If a `Decoder` is using `Flux` or `Mono` operators such as `flatMap`, `reduce`, and
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others that prefetch and cache data items internally, or is using operators such as
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`filter`, `skip`, and others that leave out items, then
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`doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release)` must be added to the
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composition chain to ensure such buffers are released prior to being discarded, possibly
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also as a result an error or cancellation signal.
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. If a `Decoder` holds on to one or more data buffers in any other way, it must
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ensure they are released when fully read, or in case an error or cancellation signals that
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take place before the cached data buffers have been read and released.
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Note that `DataBufferUtils#join` offers a safe and efficient way to aggregate a data
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buffer stream into a single data buffer. Likewise `skipUntilByteCount` and
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`takeUntilByteCount` are additional safe methods for decoders to use.
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An `Encoder` allocates data buffers that others must read (and release). So an `Encoder`
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doesn't have much to do. However an `Encoder` must take care to release a data buffer if
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a serialization error occurs while populating the buffer with data. For example:
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====
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[source,java,indent=0]
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[subs="verbatim,quotes"]
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----
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DataBuffer buffer = factory.allocateBuffer();
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boolean release = true;
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try {
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// serialize and populate buffer..
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release = false;
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}
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finally {
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if (release) {
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DataBufferUtils.release(buffer);
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}
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}
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return buffer;
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----
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====
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The consumer of an `Encoder` is responsible for releasing the data buffers it receives.
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In a WebFlux application, the output of the `Encoder` is used to write to the HTTP server
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response, or to the client HTTP request, in which case releasing the data buffers is the
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responsibility of the code writing to the server response, or to the client request.
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Note that when running on Netty, there are debugging options for
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https://github.com/netty/netty/wiki/Reference-counted-objects#troubleshooting-buffer-leaks[troubleshooting buffer leaks].

‎src/docs/asciidoc/web/webflux-websocket.adoc‎

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[[webflux-websocket-databuffer]]
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=== `DataBuffer`
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`DataBuffer` is the representation for a byte buffer in WebFlux. The Spring Core part of
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the reference has more on that in the section on
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<<core#databuffers,Data Buffers and Codecs>>. The key point to understand is that on some
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servers like Netty, byte buffers are pooled and reference counted, and must be released
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when consumed to avoid memory leaks.
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When running on Netty, applications must use `DataBufferUtils.retain(dataBuffer)` if they
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wish to hold on input data buffers in order to ensure they are not released, and
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subsequently use `DataBufferUtils.release(dataBuffer)` when the buffers are consumed.
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[[webflux-websocket-server-handshake]]
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=== Handshake
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[.small]#<<web.adoc#websocket-server-handshake,Same as in the Servlet stack>>#

‎src/docs/asciidoc/web/webflux.adoc‎

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application, while a `Decoder` can be wrapped with `DecoderHttpMessageReader`.
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* {api-spring-framework}/core/io/buffer/DataBuffer.html[`DataBuffer`] abstracts different
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byte buffer representations (e.g. Netty `ByteBuf`, `java.nio.ByteBuffer`, etc.) and is
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what all codecs work on. See <<core#databuffers, Data Buffers and Codecs>> in the
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what all codecs work on. See <<core#databuffers,Data Buffers and Codecs>> in the
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"Spring Core" section for more on this topic.
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The `spring-core` module provides `byte[]`, `ByteBuffer`, `DataBuffer`, `Resource`, and
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[[webflux-codecs-multipart]]
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==== Multipart Data
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==== Multipart
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`MultipartHttpMessageReader` and `MultipartHttpMessageWriter` support decoding and
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encoding "multipart/form-data" content. In turn `MultipartHttpMessageReader` delegates to
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a heartbeat.
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[[webflux-codecs-buffers]]
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==== `DataBuffer`
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`DataBuffer` is the representation for a byte buffer in WebFlux. The Spring Core part of
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the reference has more on that in the section on
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<<core#databuffers,Data Buffers and Codecs>>. The key point to understand is that on some
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servers like Netty, byte buffers are pooled and reference counted, and must be released
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when consumed to avoid memory leaks.
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WebFlux applications generally do not need to be concerned with such issues, unless they
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consume or produce data buffers directly, as opposed to relying on codecs to convert to
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and from higher level objects. Or unless they choose to create custom codecs. For such
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cases please review the the information in <<core#databuffers,Data Buffers and Codecs>>,
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especially the section on <<core#databuffers-using,Using DataBuffer>>.
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[[webflux-logging]]
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=== Logging

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