Improve Lazy.toString().

Added dedicated Lazy.toString() rendering the resolved value's ….toString() method but resorts to a constant [Unresolved] if it's not already resolved. An additional ….toString(Supplier<String>) allows to customize the fallback message if needed.

Fixes #2751.

src/main/java/org/springframework/data/util/Lazy.java

@@ -37,6 +37,7 @@ import org.springframework.util.ObjectUtils;
 public class Lazy<T> implements Supplier<T> {
 
 	private static final Lazy<?> EMPTY = new Lazy<>(() -> null, null, true);
+	static final String UNRESOLVED = "[Unresolved]";
 
 	private final Supplier<? extends T> supplier;
 
@@ -213,6 +214,21 @@ public class Lazy<T> implements Supplier<T> {
 		return Lazy.of(() -> function.apply(get()).get());
 	}
 
+	/**
+	 * Returns the {@link String} representation of the already resolved value or the one provided through the given
+	 * {@link Supplier} if the value has not been resolved yet.
+	 *
+	 * @param fallback must not be {@literal null}.
+	 * @return will never be {@literal null}.
+	 * @since 3.0.1
+	 */
+	public String toString(Supplier<String> fallback) {
+
+		Assert.notNull(fallback, "Fallback must not be null!");
+
+		return resolved ? toString() : fallback.get();
+	}
+
 	/**
 	 * Returns the value of the lazy evaluation.
 	 *
@@ -232,6 +248,16 @@ public class Lazy<T> implements Supplier<T> {
 		return value;
 	}
 
+	@Override
+	public String toString() {
+
+		if (!resolved) {
+			return UNRESOLVED;
+		}
+
+		return value == null ? "null" : value.toString();
+	}
+
 	@Override
 	public boolean equals(@Nullable Object o) {
 

src/test/java/org/springframework/data/util/LazyUnitTests.java

@@ -116,4 +116,35 @@ class LazyUnitTests {
 		assertThat(empty.or(reference).get()).isEqualTo(reference);
 		assertThat(empty.or(() -> reference).get()).isEqualTo(reference);
 	}
+
+	@Test // #2751
+	void doesNotResolveValueForToString() {
+
+		Supplier<Object> mock = mock(Supplier.class);
+
+		var lazy = Lazy.of(mock);
+
+		assertThat(lazy.toString()).isEqualTo(Lazy.UNRESOLVED);
+		verify(mock, never()).get();
+
+		lazy.getNullable();
+
+		assertThat(lazy.toString()).isNotEqualTo(Lazy.UNRESOLVED);
+	}
+
+	@Test // #2751
+	void usesFallbackForToStringOnUnresolvedInstance() {
+
+		Supplier<Object> mock = mock(Supplier.class);
+
+		var lazy = Lazy.of(mock);
+		var fallback = "fallback";
+
+		assertThat(lazy.toString(() -> fallback)).isEqualTo(fallback);
+		verify(mock, never()).get();
+
+		lazy.getNullable();
+
+		assertThat(lazy.toString(() -> fallback)).isNotEqualTo(fallback);
+	}
 }