GH-10083: Implement Nullability in ZK module

Related to: https://github.com/spring-projects/spring-integration/issues/10083

* Add `@org.jspecify.annotations.NullMarked` to all the packages in the ZK module
* Fix all the respective Nullability failures in the ZK module

* Fix message in assert for `candidate` in the `LeaderInitiatorFactoryBean.afterPropertiesSet()`
* Fix warning for `serialVersionUID` in the `ZookeeperLockRegistry`

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/config/CuratorFrameworkFactoryBean.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2024 the original author or authors.
+ * Copyright 2015-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -115,9 +115,7 @@ public class CuratorFrameworkFactoryBean implements FactoryBean<CuratorFramework
 		this.lifecycleLock.lock();
 		try {
 			if (!this.running) {
-				if (this.client != null) {
-					this.client.start();
-				}
+				this.client.start();
 				this.running = true;
 			}
 		}

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/config/LeaderInitiatorFactoryBean.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2023 the original author or authors.
+ * Copyright 2015-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@ package org.springframework.integration.zookeeper.config;
 import java.util.UUID;
 
 import org.apache.curator.framework.CuratorFramework;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.InitializingBean;
@@ -31,6 +32,7 @@ import org.springframework.integration.leader.event.DefaultLeaderEventPublisher;
 import org.springframework.integration.leader.event.LeaderEventPublisher;
 import org.springframework.integration.zookeeper.leader.LeaderInitiator;
 import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 
 /**
  * Creates a {@link LeaderInitiator}.
@@ -44,21 +46,22 @@ import org.springframework.util.Assert;
 public class LeaderInitiatorFactoryBean
 		implements FactoryBean<LeaderInitiator>, SmartLifecycle, InitializingBean, ApplicationEventPublisherAware {
 
-	private CuratorFramework client;
+	private @Nullable CuratorFramework client;
 
-	private Candidate candidate;
+	private @Nullable Candidate candidate;
 
-	private String path;
+	private @Nullable String path;
 
-	private LeaderInitiator leaderInitiator;
+	private @Nullable LeaderInitiator leaderInitiator;
 
 	private boolean autoStartup = true;
 
 	private int phase = Integer.MAX_VALUE - 1000; // NOSONAR magic number
 
+	@SuppressWarnings("NullAway.Init")
 	private ApplicationEventPublisher applicationEventPublisher;
 
-	private LeaderEventPublisher leaderEventPublisher;
+	private @Nullable LeaderEventPublisher leaderEventPublisher;
 
 	public LeaderInitiatorFactoryBean() {
 	}
@@ -164,21 +167,24 @@ public class LeaderInitiatorFactoryBean
 	@Override
 	public void afterPropertiesSet() {
 		if (this.leaderInitiator == null) {
-			this.leaderInitiator = new LeaderInitiator(this.client, this.candidate, this.path);
+			Assert.notNull(this.client, "The 'CuratorFramework' must be provided.");
+			Assert.notNull(this.candidate, "The 'candidate' must be provided.");
+			this.leaderInitiator =
+					StringUtils.hasText(this.path)
+							? new LeaderInitiator(this.client, this.candidate, this.path)
+							: new LeaderInitiator(this.client, this.candidate);
 			this.leaderInitiator.setPhase(this.phase);
 			this.leaderInitiator.setAutoStartup(this.autoStartup);
-			if (this.leaderEventPublisher != null) {
-				this.leaderInitiator.setLeaderEventPublisher(this.leaderEventPublisher);
-			}
-			else if (this.applicationEventPublisher != null) {
-				this.leaderInitiator.setLeaderEventPublisher(
-						new DefaultLeaderEventPublisher(this.applicationEventPublisher));
+			LeaderEventPublisher leaderEventPublisherToSet = this.leaderEventPublisher;
+			if (leaderEventPublisherToSet == null) {
+				leaderEventPublisherToSet = new DefaultLeaderEventPublisher(this.applicationEventPublisher);
 			}
+			this.leaderInitiator.setLeaderEventPublisher(leaderEventPublisherToSet);
 		}
 	}
 
 	@Override
-	public LeaderInitiator getObject() {
+	public @Nullable LeaderInitiator getObject() {
 		return this.leaderInitiator;
 	}
 

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/config/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Provides classes related to configuration.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.zookeeper.config;

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/config/xml/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Base package for zookeeper configuration.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.zookeeper.config.xml;

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/leader/LeaderInitiator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2023 the original author or authors.
+ * Copyright 2014-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,12 +28,12 @@ import org.apache.curator.framework.imps.CuratorFrameworkState;
 import org.apache.curator.framework.recipes.leader.LeaderSelector;
 import org.apache.curator.framework.recipes.leader.LeaderSelectorListenerAdapter;
 import org.apache.curator.framework.recipes.leader.Participant;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.context.SmartLifecycle;
 import org.springframework.integration.leader.Candidate;
 import org.springframework.integration.leader.Context;
 import org.springframework.integration.leader.event.LeaderEventPublisher;
-import org.springframework.util.StringUtils;
 
 /**
  * Bootstrap leadership {@link Candidate candidates}
@@ -79,7 +79,7 @@ public class LeaderInitiator implements SmartLifecycle {
 	/**
 	 * Leader event publisher if set
 	 */
-	private LeaderEventPublisher leaderEventPublisher;
+	private @Nullable LeaderEventPublisher leaderEventPublisher;
 
 	/**
 	 * @see SmartLifecycle
@@ -94,7 +94,7 @@ public class LeaderInitiator implements SmartLifecycle {
 	/**
 	 * Curator utility for selecting leaders.
 	 */
-	private volatile LeaderSelector leaderSelector;
+	private volatile @Nullable LeaderSelector leaderSelector;
 
 	/**
 	 * Flag that indicates whether the leadership election for
@@ -167,15 +167,17 @@ public class LeaderInitiator implements SmartLifecycle {
 			if (!this.running) {
 				if (this.client.getState() != CuratorFrameworkState.STARTED) {
 					// we want to do curator start here because it needs to
-					// be started before leader selector and it gets a little
+					// be started before leader selector, and it gets a little
 					// complicated to control ordering via beans so that
 					// curator is fully started.
 					this.client.start();
 				}
-				this.leaderSelector = new LeaderSelector(this.client, buildLeaderPath(), new LeaderListener());
-				this.leaderSelector.setId(this.candidate.getId());
-				this.leaderSelector.autoRequeue();
-				this.leaderSelector.start();
+				LeaderSelector leaderSelectorToSet =
+						new LeaderSelector(this.client, buildLeaderPath(), new LeaderListener());
+				leaderSelectorToSet.setId(this.candidate.getId());
+				leaderSelectorToSet.autoRequeue();
+				leaderSelectorToSet.start();
+				this.leaderSelector = leaderSelectorToSet;
 
 				this.running = true;
 				LOGGER.debug("Started LeaderInitiator");
@@ -195,7 +197,10 @@ public class LeaderInitiator implements SmartLifecycle {
 		this.lifecycleMonitor.lock();
 		try {
 			if (this.running) {
-				this.leaderSelector.close();
+				LeaderSelector leaderSelectorToClose = this.leaderSelector;
+				if (leaderSelectorToClose != null) {
+					leaderSelectorToClose.close();
+				}
 				this.running = false;
 				LOGGER.debug("Stopped LeaderInitiator");
 			}
@@ -229,7 +234,7 @@ public class LeaderInitiator implements SmartLifecycle {
 	 * @return the ZooKeeper path used for leadership election by Curator
 	 */
 	private String buildLeaderPath() {
-		String ns = StringUtils.hasText(this.namespace) ? this.namespace : DEFAULT_NAMESPACE;
+		String ns = this.namespace;
 		if (ns.charAt(0) != '/') {
 			ns = '/' + ns;
 		}
@@ -294,12 +299,16 @@ public class LeaderInitiator implements SmartLifecycle {
 
 		@Override
 		public boolean isLeader() {
-			return LeaderInitiator.this.leaderSelector.hasLeadership();
+			LeaderSelector leaderSelectorToCheck = LeaderInitiator.this.leaderSelector;
+			return leaderSelectorToCheck != null && leaderSelectorToCheck.hasLeadership();
 		}
 
 		@Override
 		public void yield() {
-			LeaderInitiator.this.leaderSelector.interruptLeadership();
+			LeaderSelector leaderSelectorToInterrupt = LeaderInitiator.this.leaderSelector;
+			if (leaderSelectorToInterrupt != null) {
+				leaderSelectorToInterrupt.interruptLeadership();
+			}
 		}
 
 		@Override
@@ -312,13 +321,17 @@ public class LeaderInitiator implements SmartLifecycle {
 		 * @return the leader.
 		 * @since 6.0.3
 		 */
-		public Participant getLeader() {
-			try {
-				return LeaderInitiator.this.leaderSelector.getLeader();
-			}
-			catch (Exception ex) {
-				throw new IllegalStateException(ex);
+		public @Nullable Participant getLeader() {
+			LeaderSelector leaderSelectorToUse = LeaderInitiator.this.leaderSelector;
+			if (leaderSelectorToUse != null) {
+				try {
+					return leaderSelectorToUse.getLeader();
+				}
+				catch (Exception ex) {
+					throw new IllegalStateException(ex);
+				}
 			}
+			return null;
 		}
 
 		/**
@@ -327,12 +340,16 @@ public class LeaderInitiator implements SmartLifecycle {
 		 * @since 6.0.3
 		 */
 		public Collection<Participant> getParticipants() {
-			try {
-				return LeaderInitiator.this.leaderSelector.getParticipants();
-			}
-			catch (Exception ex) {
-				throw new IllegalStateException(ex);
+			LeaderSelector leaderSelectorToUse = LeaderInitiator.this.leaderSelector;
+			if (leaderSelectorToUse != null) {
+				try {
+					return leaderSelectorToUse.getParticipants();
+				}
+				catch (Exception ex) {
+					throw new IllegalStateException(ex);
+				}
 			}
+			return List.of();
 		}
 
 		@Override
@@ -352,12 +369,7 @@ public class LeaderInitiator implements SmartLifecycle {
 		}
 
 		@Override
-		public String getRole() {
-			return LeaderInitiator.this.candidate.getRole();
-		}
-
-		@Override
-		public Participant getLeader() {
+		public @Nullable Participant getLeader() {
 			return null;
 		}
 

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/leader/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Temporary package until s-c-c-zookeeper is released.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.zookeeper.leader;

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/lock/ZookeeperLockRegistry.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2023 the original author or authors.
+ * Copyright 2015-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 
 package org.springframework.integration.zookeeper.lock;
 
+import java.io.Serial;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -63,7 +64,10 @@ public class ZookeeperLockRegistry implements ExpirableLockRegistry, DisposableB
 	private final Lock locksLock = new ReentrantLock();
 
 	private final Map<String, ZkLock> locks =
-			new LinkedHashMap<String, ZkLock>(16, 0.75F, true) {
+			new LinkedHashMap<>(16, 0.75F, true) {
+
+				@Serial
+				private static final long serialVersionUID = 7092378879531819061L;
 
 				@Override
 				protected boolean removeEldestEntry(Entry<String, ZkLock> eldest) {
@@ -334,9 +338,7 @@ public class ZookeeperLockRegistry implements ExpirableLockRegistry, DisposableB
 				}
 			}
 			catch (@SuppressWarnings("unused") TimeoutException e) {
-				if (future != null) {
-					future.cancel(true);
-				}
+				future.cancel(true);
 				return false;
 			}
 			catch (InterruptedException e) {

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/lock/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Provides classes related to locking.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.zookeeper.lock;

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/metadata/ZookeeperMetadataStore.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2024 the original author or authors.
+ * Copyright 2015-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,7 @@ package org.springframework.integration.zookeeper.metadata;
 
 import java.nio.charset.StandardCharsets;
 import java.util.List;
+import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.CopyOnWriteArrayList;
@@ -31,6 +32,7 @@ import org.apache.curator.framework.recipes.cache.CuratorCacheListener;
 import org.apache.curator.utils.CloseableUtils;
 import org.apache.zookeeper.KeeperException;
 import org.apache.zookeeper.data.Stat;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.context.SmartLifecycle;
 import org.springframework.integration.metadata.ListenableMetadataStore;
@@ -67,7 +69,7 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 
 	private String encoding = StandardCharsets.UTF_8.name();
 
-	private CuratorCache cache;
+	private @Nullable CuratorCache cache;
 
 	private boolean autoStartup = true;
 
@@ -79,7 +81,7 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 
 	public ZookeeperMetadataStore(CuratorFramework client) {
 		Assert.notNull(client, "Client cannot be null");
-		this.client = client;  // NOSONAR
+		this.client = client;
 	}
 
 	/**
@@ -115,7 +117,7 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 	}
 
 	@Override
-	public String putIfAbsent(String key, String value) {
+	public @Nullable String putIfAbsent(String key, String value) {
 		this.lock.lock();
 		try {
 			Assert.notNull(key, KEY_MUST_NOT_BE_NULL);
@@ -207,12 +209,13 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 	}
 
 	@Override
-	public String get(String key) {
+	public @Nullable String get(String key) {
 		this.lock.lock();
 		try {
 			Assert.notNull(key, KEY_MUST_NOT_BE_NULL);
 			Assert.state(isRunning(), "ZookeeperMetadataStore has to be started before using.");
-			return this.cache.get(getPath(key))
+			return Objects.requireNonNull(this.cache)
+					.get(getPath(key))
 					.map(currentData -> {
 						// our version is more recent than the cache
 						if (this.updateMap.containsKey(key) &&
@@ -240,7 +243,7 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 	}
 
 	@Override
-	public String remove(String key) {
+	public @Nullable String remove(String key) {
 		this.lock.lock();
 		try {
 			Assert.notNull(key, KEY_MUST_NOT_BE_NULL);
@@ -276,7 +279,7 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 	}
 
 	public String getPath(String key) {
-		return "".equals(key) ? this.root : this.root + '/' + key;
+		return key.isEmpty() ? this.root : this.root + '/' + key;
 	}
 
 	@Override
@@ -344,7 +347,7 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 		return path.replace(this.root + '/', "");
 	}
 
-	private record LocalChildData(String value, int version) {
+	private record LocalChildData(@Nullable String value, int version) {
 
 	}
 

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

@@ -2,4 +2,5 @@
  * Provides classes supporting the Zookeeper-based
  * {@link org.springframework.integration.metadata.ListenableMetadataStore}
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.zookeeper.metadata;