Add support for configuration properties binding

Create a new `Binder` class specifically designed to bind properties
from one or more `ConfigurationPropertySources` to an object.

The binder provides a replacement for `RelaxedBinder` and attempts to
fix the limitations of the previous solution.

Closes gh-8868

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/AbstractBindHandler.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2012-2017 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.context.properties.bind;
+
+import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
+import org.springframework.util.Assert;
+
+/**
+ * Abstract base class for {@link BindHandler} implementations.
+ *
+ * @author Phillip Webb
+ * @author Madhura Bhave
+ * @since 2.0.0
+ */
+public abstract class AbstractBindHandler implements BindHandler {
+
+	private final BindHandler parent;
+
+	/**
+	 * Create a new binding handler instance.
+	 */
+	public AbstractBindHandler() {
+		this(BindHandler.DEFAULT);
+	}
+
+	/**
+	 * Create a new binding handler instance with a specific parent.
+	 * @param parent the parent handler
+	 */
+	public AbstractBindHandler(BindHandler parent) {
+		Assert.notNull(parent, "Parent must not be null");
+		this.parent = parent;
+	}
+
+	@Override
+	public boolean onStart(ConfigurationPropertyName name, Bindable<?> target,
+			BindContext context) {
+		return this.parent.onStart(name, target, context);
+	}
+
+	@Override
+	public Object onSuccess(ConfigurationPropertyName name, Bindable<?> target,
+			BindContext context, Object result) {
+		return this.parent.onSuccess(name, target, context, result);
+	}
+
+	@Override
+	public Object onFailure(ConfigurationPropertyName name, Bindable<?> target,
+			BindContext context, Exception error) throws Exception {
+		return this.parent.onFailure(name, target, context, error);
+	}
+
+	@Override
+	public void onFinish(ConfigurationPropertyName name, Bindable<?> target,
+			BindContext context, Object result) throws Exception {
+		this.parent.onFinish(name, target, context, result);
+	}
+
+}

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/AggregateBinder.java

@@ -0,0 +1,130 @@
+/*
+ * Copyright 2012-2017 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.context.properties.bind;
+
+import java.util.function.Supplier;
+
+import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
+import org.springframework.core.ResolvableType;
+
+/**
+ * Internal strategy used by {@link Binder} to bind aggregates (Maps, Lists, Arrays).
+ *
+ * @param <T> the type being bound
+ * @author Phillip Webb
+ * @author Madhura Bhave
+ */
+abstract class AggregateBinder<T> {
+
+	private final BindContext context;
+
+	AggregateBinder(BindContext context) {
+		this.context = context;
+	}
+
+	/**
+	 * Perform binding for the aggregate.
+	 * @param name the configuration property name to bind
+	 * @param target the target to bind
+	 * @param itemBinder an item binder
+	 * @return the bound aggregate or null
+	 */
+	@SuppressWarnings("unchecked")
+	public final Object bind(ConfigurationPropertyName name, Bindable<?> target,
+			AggregateElementBinder itemBinder) {
+		Supplier<?> value = target.getValue();
+		Class<?> type = (value == null ? target.getType().resolve()
+				: ResolvableType.forClass(AggregateBinder.class, getClass())
+						.resolveGeneric());
+		Object result = bind(name, target, itemBinder, type);
+		if (result == null || value == null || value.get() == null) {
+			return result;
+		}
+		return merge((T) value.get(), (T) result);
+	}
+
+	/**
+	 * Perform the actual aggregate binding.
+	 * @param name the configuration property name to bind
+	 * @param target the target to bind
+	 * @param elementBinder an element binder
+	 * @param type the aggregate actual type to use
+	 * @return the bound result
+	 */
+	protected abstract Object bind(ConfigurationPropertyName name, Bindable<?> target,
+			AggregateElementBinder elementBinder, Class<?> type);
+
+	/**
+	 * Merge any additional elements into the existing aggregate.
+	 * @param existing the existing value
+	 * @param additional the additional elements to merge
+	 * @return the merged result
+	 */
+	protected abstract T merge(T existing, T additional);
+
+	/**
+	 * Return the context being used by this binder.
+	 * @return the context
+	 */
+	protected final BindContext getContext() {
+		return this.context;
+	}
+
+	/**
+	 * Roll up the given name to the first element below the root. For example a name of
+	 * {@code foo.bar.baz} rolled up to the root {@code foo} would be {@code foo.bar}.
+	 * @param name the name to roll up
+	 * @param root the root name
+	 * @return the rolled up name or {@code null}
+	 */
+	protected final ConfigurationPropertyName rollUp(ConfigurationPropertyName name,
+			ConfigurationPropertyName root) {
+		while (name != null && (name.getParent() != null)
+				&& (!root.equals(name.getParent()))) {
+			name = name.getParent();
+		}
+		return name;
+	}
+
+	/**
+	 * Internal class used to supply the aggregate and cache the value.
+	 * @param <T> The aggregate type
+	 */
+	protected static class AggregateSupplier<T> {
+
+		private final Supplier<T> supplier;
+
+		private T supplied;
+
+		public AggregateSupplier(Supplier<T> supplier) {
+			this.supplier = supplier;
+		}
+
+		public T get() {
+			if (this.supplied == null) {
+				this.supplied = this.supplier.get();
+			}
+			return this.supplied;
+		}
+
+		public boolean wasSupplied() {
+			return this.supplied != null;
+		}
+
+	}
+
+}

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/AggregateElementBinder.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2012-2017 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.context.properties.bind;
+
+import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
+import org.springframework.boot.context.properties.source.ConfigurationPropertySource;
+
+/**
+ * Binder that can be used by {@link AggregateBinder} implementations to recursively bind
+ * elements.
+ *
+ * @author Phillip Webb
+ * @author Madhura Bhave
+ */
+@FunctionalInterface
+interface AggregateElementBinder {
+
+	/**
+	 * Bind the given name to a target bindable.
+	 * @param name the name to bind
+	 * @param target the target bindable
+	 * @return a bound object or {@code null}
+	 */
+	default Object bind(ConfigurationPropertyName name, Bindable<?> target) {
+		return bind(name, target, null);
+	}
+
+	/**
+	 * Bind the given name to a target bindable using optionally limited to a single
+	 * source.
+	 * @param name the name to bind
+	 * @param target the target bindable
+	 * @param source the source of the elements or {@code null} to use all sources
+	 * @return a bound object or {@code null}
+	 */
+	Object bind(ConfigurationPropertyName name, Bindable<?> target,
+			ConfigurationPropertySource source);
+
+}

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/ArrayBinder.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2012-2017 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.context.properties.bind;
+
+import java.lang.reflect.Array;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
+import org.springframework.core.ResolvableType;
+
+/**
+ * {@link AggregateBinder} for arrays.
+ *
+ * @author Phillip Webb
+ * @author Madhura Bhave
+ */
+class ArrayBinder extends IndexedElementsBinder<Object> {
+
+	ArrayBinder(BindContext context) {
+		super(context);
+	}
+
+	@Override
+	protected Object bind(ConfigurationPropertyName name, Bindable<?> target,
+			AggregateElementBinder elementBinder, Class<?> type) {
+		IndexedCollectionSupplier collection = new IndexedCollectionSupplier(
+				ArrayList::new);
+		ResolvableType elementType = target.getType().getComponentType();
+		bindIndexed(name, target, elementBinder, collection, target.getType(),
+				elementType);
+		if (collection.wasSupplied()) {
+			List<Object> list = (List<Object>) collection.get();
+			Object array = Array.newInstance(elementType.resolve(), list.size());
+			for (int i = 0; i < list.size(); i++) {
+				Array.set(array, i, list.get(i));
+			}
+			return array;
+		}
+		return null;
+	}
+
+	@Override
+	protected Object merge(Object existing, Object additional) {
+		return additional;
+	}
+
+}

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/BeanBinder.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2012-2017 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.context.properties.bind;
+
+import org.springframework.boot.context.properties.source.ConfigurationPropertySource;
+
+/**
+ * Internal strategy used by {@link Binder} to bind beans.
+ *
+ * @author Phillip Webb
+ * @author Madhura Bhave
+ */
+interface BeanBinder {
+
+	/**
+	 * Return a bound bean instance or {@code null} if the {@link BeanBinder} does not
+	 * support the specified {@link Bindable}.
+	 * @param target the binable to bind
+	 * @param hasKnownBindableProperties if this binder has known bindable elements. If
+	 * names from underlying {@link ConfigurationPropertySource} cannot be iterated this
+	 * method can be {@code false}, even though binding may ultimately succeed.
+	 * @param propertyBinder property binder
+	 * @param <T> The source type
+	 * @return a bound instance or {@code null}
+	 */
+	<T> T bind(Bindable<T> target, boolean hasKnownBindableProperties,
+			BeanPropertyBinder propertyBinder);
+
+}