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/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);
+	}
 }