Add default methods to Session

Fixes gh-819

spring-session-core/src/main/java/org/springframework/session/Session.java

@@ -16,6 +16,8 @@
 
 package org.springframework.session;
 
+import org.springframework.util.Assert;
+
 import java.time.Duration;
 import java.time.Instant;
 import java.util.Set;
@@ -48,6 +50,33 @@ public interface Session {
 	 */
 	<T> T getAttribute(String attributeName);
 
+	/**
+	 * Return the session attribute value or if not present raise an
+	 * {@link IllegalArgumentException}.
+	 * @param name the attribute name
+	 * @param <T> the attribute type
+	 * @return the attribute value
+	 */
+	@SuppressWarnings("unchecked")
+	default <T> T getRequiredAttribute(String name) {
+		T result = getAttribute(name);
+		Assert.notNull(result, "Required attribute '" + name + "' is missing.");
+		return result;
+	}
+
+	/**
+	 * Return the session attribute value, or a default, fallback value.
+	 * @param name the attribute name
+	 * @param defaultValue a default value to return instead
+	 * @param <T> the attribute type
+	 * @return the attribute value
+	 */
+	@SuppressWarnings("unchecked")
+	default <T> T getAttributeOrDefault(String name, T defaultValue) {
+		T result = getAttribute(name);
+		return result == null ? defaultValue : result;
+	}
+
 	/**
 	 * Gets the attribute names that have a value associated with it. Each value can be
 	 * passed into {@link org.springframework.session.Session#getAttribute(String)} to

spring-session-core/src/test/java/org/springframework/session/MapSessionTests.java

@@ -40,6 +40,47 @@ public class MapSessionTests {
 		new MapSession((Session) null);
 	}
 
+	@Test
+	public void getAttributeWhenNullThenNull() {
+		String result = this.session.getAttribute("attrName");
+		assertThat(result).isNull();
+	}
+
+	@Test
+	public void getAttributeOrDefaultWhenNullThenDefaultValue() {
+		String defaultValue = "default";
+		String result = this.session.getAttributeOrDefault("attrName", defaultValue);
+		assertThat(result).isEqualTo(defaultValue);
+	}
+
+	@Test
+	public void getAttributeOrDefaultWhenNotNullThenDefaultValue() {
+		String defaultValue = "default";
+		String attrValue = "value";
+		String attrName = "attrName";
+		this.session.setAttribute(attrName, attrValue);
+
+		String result = this.session.getAttributeOrDefault(attrName, defaultValue);
+
+		assertThat(result).isEqualTo(attrValue);
+	}
+
+	@Test(expected = IllegalArgumentException.class)
+	public void getRequiredAttributeWhenNullThenException() {
+		this.session.getRequiredAttribute("attrName");
+	}
+
+	@Test
+	public void getRequiredAttributeWhenNotNullThenReturns() {
+		String attrValue = "value";
+		String attrName = "attrName";
+		this.session.setAttribute(attrName, attrValue);
+
+		String result = this.session.getRequiredAttribute("attrName");
+
+		assertThat(result).isEqualTo(attrValue);
+	}
+
 	/**
 	 * Ensure conforms to the javadoc of {@link Session}
 	 */