Add missing registration convenience methods

Update `BootstrapContext` with convenience methods that help if the
type has not been registered.

Closes gh-23438

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/BootstrapContext.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot;
 
+import java.util.function.Supplier;
+
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.env.Environment;
 
@@ -32,8 +34,8 @@ import org.springframework.core.env.Environment;
 public interface BootstrapContext {
 
 	/**
-	 * Return an instance from the context, creating it if it hasn't been accessed
-	 * previously.
+	 * Return an instance from the context if the type has been registered. The instance
+	 * will be created it if it hasn't been accessed previously.
 	 * @param <T> the instance type
 	 * @param type the instance type
 	 * @return the instance managed by the context
@@ -41,4 +43,45 @@ public interface BootstrapContext {
 	 */
 	<T> T get(Class<T> type) throws IllegalStateException;
 
+	/**
+	 * Return an instance from the context if the type has been registered. The instance
+	 * will be created it if it hasn't been accessed previously.
+	 * @param <T> the instance type
+	 * @param type the instance type
+	 * @param other the instance to use if the type has not been registered
+	 * @return the instance
+	 */
+	<T> T getOrElse(Class<T> type, T other);
+
+	/**
+	 * Return an instance from the context if the type has been registered. The instance
+	 * will be created it if it hasn't been accessed previously.
+	 * @param <T> the instance type
+	 * @param type the instance type
+	 * @param other a supplier for the instance to use if the type has not been registered
+	 * @return the instance
+	 */
+	<T> T getOrElseSupply(Class<T> type, Supplier<T> other);
+
+	/**
+	 * Return an instance from the context if the type has been registered. The instance
+	 * will be created it if it hasn't been accessed previously.
+	 * @param <T> the instance type
+	 * @param <X> the exception to throw if the type is not registered
+	 * @param type the instance type
+	 * @param exceptionSupplier the supplier which will return the exception to be thrown
+	 * @return the instance managed by the context
+	 * @throws X if the type has not been registered
+	 * @throws IllegalStateException if the type has not been registered
+	 */
+	<T, X extends Throwable> T getOrElseThrow(Class<T> type, Supplier<? extends X> exceptionSupplier) throws X;
+
+	/**
+	 * Return if a registration exists for the given type.
+	 * @param <T> the instance type
+	 * @param type the instance type
+	 * @return {@code true} if the type has already been registered
+	 */
+	<T> boolean isRegistered(Class<T> type);
+
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/DefaultBootstrapContext.java

@@ -18,6 +18,7 @@ package org.springframework.boot;
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.function.Supplier;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationListener;
@@ -83,18 +84,42 @@ public class DefaultBootstrapContext implements ConfigurableBootstrapContext {
 	}
 
 	@Override
-	@SuppressWarnings("unchecked")
 	public <T> T get(Class<T> type) throws IllegalStateException {
+		return getOrElseThrow(type, () -> new IllegalStateException(type.getName() + " has not been registered"));
+	}
+
+	@Override
+	public <T> T getOrElse(Class<T> type, T other) {
+		return getOrElseSupply(type, () -> other);
+	}
+
+	@Override
+	public <T> T getOrElseSupply(Class<T> type, Supplier<T> other) {
 		synchronized (this.instanceSuppliers) {
 			InstanceSupplier<?> instanceSupplier = this.instanceSuppliers.get(type);
-			Assert.state(instanceSupplier != null, () -> type.getName() + " has not been registered");
-			T instance = (T) this.instances.get(type);
-			if (instance == null) {
-				instance = (T) instanceSupplier.get(this);
-				this.instances.put(type, instance);
+			return (instanceSupplier != null) ? getInstance(type, instanceSupplier) : other.get();
+		}
+	}
+
+	@Override
+	public <T, X extends Throwable> T getOrElseThrow(Class<T> type, Supplier<? extends X> exceptionSupplier) throws X {
+		synchronized (this.instanceSuppliers) {
+			InstanceSupplier<?> instanceSupplier = this.instanceSuppliers.get(type);
+			if (instanceSupplier == null) {
+				throw exceptionSupplier.get();
 			}
-			return instance;
+			return getInstance(type, instanceSupplier);
+		}
+	}
+
+	@SuppressWarnings("unchecked")
+	private <T> T getInstance(Class<T> type, InstanceSupplier<?> instanceSupplier) {
+		T instance = (T) this.instances.get(type);
+		if (instance == null) {
+			instance = (T) instanceSupplier.get(this);
+			this.instances.put(type, instance);
 		}
+		return instance;
 	}
 
 	/**

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/DefaultBootstrapContextTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot;
 
+import java.io.IOException;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import org.assertj.core.api.AbstractAssert;
@@ -29,6 +30,7 @@ import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.support.StaticApplicationContext;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIOException;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
@@ -150,6 +152,63 @@ class DefaultBootstrapContextTests {
 		assertThat(this.context.get(Integer.class)).isEqualTo(0);
 	}
 
+	@Test
+	void getOrElseWhenNoRegistrationReturnsOther() {
+		this.context.register(Number.class, InstanceSupplier.of(1));
+		assertThat(this.context.getOrElse(Long.class, -1L)).isEqualTo(-1);
+	}
+
+	@Test
+	void getOrElseWhenRegisteredAsNullReturnsNull() {
+		this.context.register(Number.class, InstanceSupplier.of(null));
+		assertThat(this.context.getOrElse(Number.class, -1)).isNull();
+	}
+
+	@Test
+	void getOrElseCreatesReturnsOnlyOneInstance() {
+		this.context.register(Integer.class, InstanceSupplier.from(this.counter::getAndIncrement));
+		assertThat(this.context.getOrElse(Integer.class, -1)).isEqualTo(0);
+		assertThat(this.context.getOrElse(Integer.class, -1)).isEqualTo(0);
+	}
+
+	@Test
+	void getOrElseSupplyWhenNoRegistrationReturnsSupplied() {
+		this.context.register(Number.class, InstanceSupplier.of(1));
+		assertThat(this.context.getOrElseSupply(Long.class, () -> -1L)).isEqualTo(-1);
+	}
+
+	@Test
+	void getOrElseSupplyWhenRegisteredAsNullReturnsNull() {
+		this.context.register(Number.class, InstanceSupplier.of(null));
+		assertThat(this.context.getOrElseSupply(Number.class, () -> -1L)).isNull();
+	}
+
+	@Test
+	void getOrElseSupplyCreatesOnlyOneInstance() {
+		this.context.register(Integer.class, InstanceSupplier.from(this.counter::getAndIncrement));
+		assertThat(this.context.getOrElseSupply(Integer.class, () -> -1)).isEqualTo(0);
+		assertThat(this.context.getOrElseSupply(Integer.class, () -> -1)).isEqualTo(0);
+	}
+
+	@Test
+	void getOrElseThrowWhenNoRegistrationThrowsSuppliedException() {
+		this.context.register(Number.class, InstanceSupplier.of(1));
+		assertThatIOException().isThrownBy(() -> this.context.getOrElseThrow(Long.class, IOException::new));
+	}
+
+	@Test
+	void getOrElseThrowWhenRegisteredAsNullReturnsNull() {
+		this.context.register(Number.class, InstanceSupplier.of(null));
+		assertThat(this.context.getOrElseThrow(Number.class, RuntimeException::new)).isNull();
+	}
+
+	@Test
+	void getOrElseThrowCreatesOnlyOneInstance() {
+		this.context.register(Integer.class, InstanceSupplier.from(this.counter::getAndIncrement));
+		assertThat(this.context.getOrElseThrow(Integer.class, RuntimeException::new)).isEqualTo(0);
+		assertThat(this.context.getOrElseThrow(Integer.class, RuntimeException::new)).isEqualTo(0);
+	}
+
 	@Test
 	void closeMulticastsEventToListeners() {
 		TestCloseListener listener = new TestCloseListener();