GH-10083: Add nullability to Core metadata package

Related to: #10083

* `ConcurrentMetadataStore`: Added `@Nullable` annotation to `putIfAbsent` method return type for better null safety.
* `MetadataStore`: Added `@Nullable` annotations to `get` and `remove` method return types to indicate they can return null.
* `PropertiesPersistingMetadataStore`: Removed explicit null checks with `Assert.notNull()` calls and added `@Nullable` annotations to method signatures while simplifying `saveMetadata` logic.
* `SimpleMetadataStore`: Added `@Nullable` annotations to `get`, `remove`, and `putIfAbsent` method return types for consistency with interface.
* `MetadataStoreSelector`: Added `Assert.state()` calls to validate that `keyStrategy` and `valueStrategy` do not return null values.

Author: cppwfs

spring-integration-core/src/main/java/org/springframework/integration/metadata/ConcurrentMetadataStore.java

@@ -16,10 +16,13 @@
 
 package org.springframework.integration.metadata;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Supports atomic updates to values in the store.
  *
  * @author Gary Russell
+ * @author Glenn Renfro
  * @since 4.0
  *
  */
@@ -32,6 +35,7 @@ public interface ConcurrentMetadataStore extends MetadataStore {
 	 * @param value The value.
 	 * @return null if successful, the old value otherwise.
 	 */
+	@Nullable
 	String putIfAbsent(String key, String value);
 
 	/**

spring-integration-core/src/main/java/org/springframework/integration/metadata/MetadataStore.java

@@ -16,6 +16,8 @@
 
 package org.springframework.integration.metadata;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.jmx.export.annotation.ManagedAttribute;
 import org.springframework.jmx.export.annotation.ManagedResource;
 
@@ -27,6 +29,7 @@
  * @author Oleg Zhurakousky
  * @author Mark Fisher
  * @author Gary Russell
+ * @author Glenn Renfro
  * @since 2.0
  */
 @ManagedResource
@@ -47,7 +50,7 @@ public interface MetadataStore {
 	 * @return The value.
 	 */
 	@ManagedAttribute
-	String get(String key);
+	@Nullable String get(String key);
 
 	/**
 	 * Remove a value for the given key from this MetadataStore.
@@ -56,6 +59,6 @@ public interface MetadataStore {
 	 *         null if there was no mapping for key.
 	 */
 	@ManagedAttribute
-	String remove(String key);
+	@Nullable String remove(String key);
 
 }

spring-integration-core/src/main/java/org/springframework/integration/metadata/PropertiesPersistingMetadataStore.java

@@ -31,6 +31,7 @@
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.beans.factory.InitializingBean;
@@ -51,14 +52,13 @@
  * @author Mark Fisher
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Glenn Renfro
  *
  * @since 2.0
  */
 public class PropertiesPersistingMetadataStore implements ConcurrentMetadataStore, InitializingBean, DisposableBean,
 		Closeable, Flushable {
 
-	private static final String KEY_CANNOT_BE_NULL = "'key' cannot be null";
-
 	private final Log logger = LogFactory.getLog(getClass());
 
 	private final Properties metadata = new Properties();
@@ -71,6 +71,7 @@ public class PropertiesPersistingMetadataStore implements ConcurrentMetadataStor
 
 	private String fileName = "metadata-store.properties";
 
+	@SuppressWarnings("NullAway.Init")
 	private File file;
 
 	private volatile boolean dirty;
@@ -115,8 +116,6 @@ public void afterPropertiesSet() {
 
 	@Override
 	public void put(String key, String value) {
-		Assert.notNull(key, KEY_CANNOT_BE_NULL);
-		Assert.notNull(value, "'value' cannot be null");
 		Lock lock = this.lockRegistry.obtain(key);
 		lock.lock();
 		try {
@@ -129,8 +128,7 @@ public void put(String key, String value) {
 	}
 
 	@Override
-	public String get(String key) {
-		Assert.notNull(key, KEY_CANNOT_BE_NULL);
+	public @Nullable String get(String key) {
 		Lock lock = this.lockRegistry.obtain(key);
 		lock.lock();
 		try {
@@ -142,8 +140,7 @@ public String get(String key) {
 	}
 
 	@Override
-	public String remove(String key) {
-		Assert.notNull(key, KEY_CANNOT_BE_NULL);
+	public @Nullable String remove(String key) {
 		Lock lock = this.lockRegistry.obtain(key);
 		lock.lock();
 		try {
@@ -156,9 +153,7 @@ public String remove(String key) {
 	}
 
 	@Override
-	public String putIfAbsent(String key, String value) {
-		Assert.notNull(key, KEY_CANNOT_BE_NULL);
-		Assert.notNull(value, "'value' cannot be null");
+	public @Nullable String putIfAbsent(String key, String value) {
 		Lock lock = this.lockRegistry.obtain(key);
 		lock.lock();
 		try {
@@ -179,9 +174,6 @@ public String putIfAbsent(String key, String value) {
 
 	@Override
 	public boolean replace(String key, String oldValue, String newValue) {
-		Assert.notNull(key, KEY_CANNOT_BE_NULL);
-		Assert.notNull(oldValue, "'oldValue' cannot be null");
-		Assert.notNull(newValue, "'newValue' cannot be null");
 		Lock lock = this.lockRegistry.obtain(key);
 		lock.lock();
 		try {
@@ -216,7 +208,7 @@ public void destroy() {
 	}
 
 	private void saveMetadata() {
-		if (this.file == null || !this.dirty) {
+		if (!this.dirty) {
 			return;
 		}
 		this.dirty = false;

spring-integration-core/src/main/java/org/springframework/integration/metadata/SimpleMetadataStore.java

@@ -19,6 +19,8 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 
 /**
@@ -29,6 +31,7 @@
  * @author Mark Fisher
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Glenn Renfro
  * @since 2.0
  */
 public class SimpleMetadataStore implements ConcurrentMetadataStore {
@@ -59,17 +62,17 @@ public void put(String key, String value) {
 	}
 
 	@Override
-	public String get(String key) {
+	public @Nullable String get(String key) {
 		return this.metadata.get(key);
 	}
 
 	@Override
-	public String remove(String key) {
+	public @Nullable String remove(String key) {
 		return this.metadata.remove(key);
 	}
 
 	@Override
-	public String putIfAbsent(String key, String value) {
+	public @Nullable String putIfAbsent(String key, String value) {
 		return this.metadata.putIfAbsent(key, value);
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/metadata/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Provides classes supporting metadata stores.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.metadata;

spring-integration-core/src/main/java/org/springframework/integration/selector/MetadataStoreSelector.java

@@ -54,6 +54,7 @@
  * @author Artem Bilan
  * @author Gary Russell
  * @author Christian Tzolov
+ * @author Glenn Renfro
  *
  * @since 4.1
  */
@@ -114,10 +115,13 @@ public MetadataStoreSelector compareValues(@Nullable BiPredicate<String, String>
 	@Override
 	public boolean accept(Message<?> message) {
 		String key = this.keyStrategy.processMessage(message);
+		Assert.state(key != null, () -> "The keyStrategy.processMessage must not return null.");
+
 		Long timestamp = message.getHeaders().getTimestamp();
 		String value = (this.valueStrategy != null)
 				? this.valueStrategy.processMessage(message)
 				: (timestamp == null ? "0" : Long.toString(timestamp));
+		Assert.state(value != null, () -> "The valueStrategy.processMessage must not return null.");
 
 		BiPredicate<String, String> predicate = this.compareValues;
 		if (predicate == null) {