Add support for DataSize

This commit adds support for Spring Framework's `DataSize` allowing to
express a size in bytes and other convenient units.

Similar to the `Duration` support introduced previously, this commit
adds transparent binding support as well as detection of default values
in `@ConfigurationProperties`-annotated object.

Closes gh-13974

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -1308,6 +1308,45 @@ while supporting a much richer format.
 
 
 
+[[boot-features-external-config-conversion-datasize]]
+===== Converting Data Sizes
+Spring Framework has a `DataSize` value type that allows to express size in bytes. If you
+expose a `DataSize` property, the following formats in application properties are
+available:
+
+* A regular `long` representation (using bytes as the default unit unless a
+`@DataSizeUnit` has been specified)
+* A more readable format where the value and the unit are coupled (e.g. `10MB` means 10
+megabytes)
+
+Consider the following example:
+
+[source,java,indent=0]
+----
+include::{code-examples}/context/properties/bind/AppIoProperties.java[tag=example]
+----
+
+To specify a buffer size of 10 megabytes, `10` and `10MB` are equivalent. A size threshold
+of 256 bytes can be specified as `256` or `256B`.
+
+You can also use any of the supported unit. These are:
+
+* `B` for bytes
+* `KB` for kilobytes
+* `MB` for megabytes
+* `GB` for gigabytes
+* `TB` for terabytes
+
+The default unit is bytes and can be overridden using `@DataSizeUnit` as illustrated
+in the sample above.
+
+TIP: If you are upgrading from a previous version that is simply using `Long` to express
+the size, make sure to define the unit (using `@DataSizeUnit`) if it isn't bytes alongside
+the switch to `DataSize`. Doing so gives a transparent upgrade path while supporting a
+much richer format.
+
+
+
 [[boot-features-external-config-validation]]
 ==== @ConfigurationProperties Validation
 Spring Boot attempts to validate `@ConfigurationProperties` classes whenever they are

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/context/properties/bind/AppIoProperties.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.context.properties.bind;
+
+import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.boot.convert.DataSizeUnit;
+import org.springframework.util.unit.DataSize;
+import org.springframework.util.unit.DataUnit;
+
+/**
+ * A {@link ConfigurationProperties} example that uses {@link DataSize}.
+ *
+ * @author Stephane Nicoll
+ */
+// tag::example[]
+@ConfigurationProperties("app.io")
+public class AppIoProperties {
+
+	@DataSizeUnit(DataUnit.MEGABYTES)
+	private DataSize bufferSize = DataSize.ofMegaBytes(2);
+
+	private DataSize sizeThreshold = DataSize.ofBytes(512);
+
+	public DataSize getBufferSize() {
+		return this.bufferSize;
+	}
+
+	public void setBufferSize(DataSize bufferSize) {
+		this.bufferSize = bufferSize;
+	}
+
+	public DataSize getSizeThreshold() {
+		return this.sizeThreshold;
+	}
+
+	public void setSizeThreshold(DataSize sizeThreshold) {
+		this.sizeThreshold = sizeThreshold;
+	}
+
+}
+// end::example[]