Introduce computeAttribute() in AttributeAccessor

This commit introduces computeAttribute() as an interface default method
in the AttributeAccessor API. This serves as a convenience analogous to
the computeIfAbsent() method in java.util.Map.

Closes gh-26281

Author: sbrannen

spring-core/src/main/java/org/springframework/core/AttributeAccessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -16,20 +16,24 @@
 
 package org.springframework.core;
 
+import java.util.function.Function;
+
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 
 /**
  * Interface defining a generic contract for attaching and accessing metadata
  * to/from arbitrary objects.
  *
  * @author Rob Harrop
+ * @author Sam Brannen
  * @since 2.0
  */
 public interface AttributeAccessor {
 
 	/**
 	 * Set the attribute defined by {@code name} to the supplied {@code value}.
-	 * If {@code value} is {@code null}, the attribute is {@link #removeAttribute removed}.
+	 * <p>If {@code value} is {@code null}, the attribute is {@link #removeAttribute removed}.
 	 * <p>In general, users should take care to prevent overlaps with other
 	 * metadata attributes by using fully-qualified names, perhaps using
 	 * class or package names as prefix.
@@ -40,16 +44,48 @@ public interface AttributeAccessor {
 
 	/**
 	 * Get the value of the attribute identified by {@code name}.
-	 * Return {@code null} if the attribute doesn't exist.
+	 * <p>Return {@code null} if the attribute doesn't exist.
 	 * @param name the unique attribute key
 	 * @return the current value of the attribute, if any
 	 */
 	@Nullable
 	Object getAttribute(String name);
 
+	/**
+	 * Compute a new value for the attribute identified by {@code name} if
+	 * necessary and {@linkplain #setAttribute set} the new value in this
+	 * {@code AttributeAccessor}.
+	 * <p>If a value for the attribute identified by {@code name} already exists
+	 * in this {@code AttributeAccessor}, the existing value will be returned
+	 * without applying the supplied compute function.
+	 * <p>The default implementation of this method is not thread safe but can
+	 * overridden by concrete implementations of this interface.
+	 * @param <T> the type of the attribute value
+	 * @param name the unique attribute key
+	 * @param computeFunction a function that computes a new value for the attribute
+	 * name; the function must not return a {@code null} value
+	 * @return the existing value or newly computed value for the named attribute
+	 * @see #getAttribute(String)
+	 * @see #setAttribute(String, Object)
+	 * @since 5.3.3
+	 */
+	@SuppressWarnings("unchecked")
+	default <T> T computeAttribute(String name, Function<String, T> computeFunction) {
+		Assert.notNull(name, "Name must not be null");
+		Assert.notNull(computeFunction, "Compute function must not be null");
+		Object value = getAttribute(name);
+		if (value == null) {
+			value = computeFunction.apply(name);
+			Assert.state(value != null,
+					() -> String.format("Compute function must not return null for attribute named '%s'", name));
+			setAttribute(name, value);
+		}
+		return (T) value;
+	}
+
 	/**
 	 * Remove the attribute identified by {@code name} and return its value.
-	 * Return {@code null} if no attribute under {@code name} is found.
+	 * <p>Return {@code null} if no attribute under {@code name} is found.
 	 * @param name the unique attribute key
 	 * @return the last value of the attribute, if any
 	 */
@@ -58,7 +94,7 @@ public interface AttributeAccessor {
 
 	/**
 	 * Return {@code true} if the attribute identified by {@code name} exists.
-	 * Otherwise return {@code false}.
+	 * <p>Otherwise return {@code false}.
 	 * @param name the unique attribute key
 	 */
 	boolean hasAttribute(String name);

spring-core/src/main/java/org/springframework/core/AttributeAccessorSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -19,6 +19,7 @@
 import java.io.Serializable;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.function.Function;
 
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -32,6 +33,7 @@
  *
  * @author Rob Harrop
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 2.0
  */
 @SuppressWarnings("serial")
@@ -59,6 +61,17 @@ public Object getAttribute(String name) {
 		return this.attributes.get(name);
 	}
 
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> T computeAttribute(String name, Function<String, T> computeFunction) {
+		Assert.notNull(name, "Name must not be null");
+		Assert.notNull(computeFunction, "Compute function must not be null");
+		Object value = this.attributes.computeIfAbsent(name, computeFunction);
+		Assert.state(value != null,
+				() -> String.format("Compute function must not return null for attribute named '%s'", name));
+		return (T) value;
+	}
+
 	@Override
 	@Nullable
 	public Object removeAttribute(String name) {

spring-core/src/test/java/org/springframework/core/AttributeAccessorSupportTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -17,53 +17,75 @@
 package org.springframework.core;
 
 import java.util.Arrays;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Function;
 
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
+ * Unit tests for {@link AttributeAccessorSupport}.
+ *
  * @author Rob Harrop
  * @author Sam Brannen
  * @since 2.0
  */
 class AttributeAccessorSupportTests {
 
-	private static final String NAME = "foo";
+	private static final String NAME = "name";
+
+	private static final String VALUE = "value";
 
-	private static final String VALUE = "bar";
+	private final AttributeAccessor attributeAccessor = new SimpleAttributeAccessorSupport();
 
-	private AttributeAccessor attributeAccessor = new SimpleAttributeAccessorSupport();
 
 	@Test
-	void setAndGet() throws Exception {
+	void setAndGet() {
 		this.attributeAccessor.setAttribute(NAME, VALUE);
 		assertThat(this.attributeAccessor.getAttribute(NAME)).isEqualTo(VALUE);
 	}
 
 	@Test
-	void setAndHas() throws Exception {
+	void setAndHas() {
 		assertThat(this.attributeAccessor.hasAttribute(NAME)).isFalse();
 		this.attributeAccessor.setAttribute(NAME, VALUE);
 		assertThat(this.attributeAccessor.hasAttribute(NAME)).isTrue();
 	}
 
 	@Test
-	void remove() throws Exception {
+	void computeAttribute() {
+		AtomicInteger atomicInteger = new AtomicInteger();
+		Function<String, String> computeFunction = name -> "computed-" + atomicInteger.incrementAndGet();
+
+		assertThat(this.attributeAccessor.hasAttribute(NAME)).isFalse();
+		this.attributeAccessor.computeAttribute(NAME, computeFunction);
+		assertThat(this.attributeAccessor.getAttribute(NAME)).isEqualTo("computed-1");
+		this.attributeAccessor.computeAttribute(NAME, computeFunction);
+		assertThat(this.attributeAccessor.getAttribute(NAME)).isEqualTo("computed-1");
+
+		this.attributeAccessor.removeAttribute(NAME);
+		assertThat(this.attributeAccessor.hasAttribute(NAME)).isFalse();
+		this.attributeAccessor.computeAttribute(NAME, computeFunction);
+		assertThat(this.attributeAccessor.getAttribute(NAME)).isEqualTo("computed-2");
+	}
+
+	@Test
+	void remove() {
 		assertThat(this.attributeAccessor.hasAttribute(NAME)).isFalse();
 		this.attributeAccessor.setAttribute(NAME, VALUE);
 		assertThat(this.attributeAccessor.removeAttribute(NAME)).isEqualTo(VALUE);
 		assertThat(this.attributeAccessor.hasAttribute(NAME)).isFalse();
 	}
 
 	@Test
-	void attributeNames() throws Exception {
+	void attributeNames() {
 		this.attributeAccessor.setAttribute(NAME, VALUE);
 		this.attributeAccessor.setAttribute("abc", "123");
 		String[] attributeNames = this.attributeAccessor.attributeNames();
 		Arrays.sort(attributeNames);
-		assertThat(Arrays.binarySearch(attributeNames, NAME) > -1).isTrue();
-		assertThat(Arrays.binarySearch(attributeNames, "abc") > -1).isTrue();
+		assertThat(Arrays.binarySearch(attributeNames, "abc")).isEqualTo(0);
+		assertThat(Arrays.binarySearch(attributeNames, NAME)).isEqualTo(1);
 	}
 
 	@SuppressWarnings("serial")

spring-test/src/main/java/org/springframework/test/context/support/DefaultTestContext.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -19,6 +19,7 @@
 import java.lang.reflect.Method;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Function;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
@@ -200,6 +201,17 @@ public Object getAttribute(String name) {
 		return this.attributes.get(name);
 	}
 
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> T computeAttribute(String name, Function<String, T> computeFunction) {
+		Assert.notNull(name, "Name must not be null");
+		Assert.notNull(computeFunction, "Compute function must not be null");
+		Object value = this.attributes.computeIfAbsent(name, computeFunction);
+		Assert.state(value != null,
+				() -> String.format("Compute function must not return null for attribute named '%s'", name));
+		return (T) value;
+	}
+
 	@Override
 	@Nullable
 	public Object removeAttribute(String name) {