Introduce a minimal retry functionality as a core framework feature

This commit introduces a minimal core retry feature. It is inspired
by Spring Retry, but redesigned and trimmed to the bare minimum to
cover most cases.

Author: fmbenhassine

spring-core/src/main/java/org/springframework/core/retry/BackOffPolicy.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.core.retry;
+
+import java.time.Duration;
+
+/**
+ * Strategy interface to define how to calculate the backoff policy.
+ *
+ * @author Mahmoud Ben Hassine
+ * @since 7.0
+ */
+public interface BackOffPolicy {
+
+    /**
+     * Signal how long to backoff before the next retry attempt.
+     *
+     * @return the duration to wait for before the next retry attempt
+     */
+    Duration backOff();
+
+}

spring-core/src/main/java/org/springframework/core/retry/RetryListener.java

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+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.core.retry;
+
+import org.springframework.core.retry.support.listener.CompositeRetryListener;
+
+/**
+ * An extension point that allows to inject code during key retry phases.
+ *
+ * <p>Typically registered in a {@link RetryTemplate}, and can be composed using
+ * a {@link CompositeRetryListener}.
+ *
+ * @author Mahmoud Ben Hassine
+ * @since 7.0
+ * @see CompositeRetryListener
+ */
+public interface RetryListener {
+
+	/**
+	 * Called before every retry attempt.
+	 */
+    default void beforeRetry() {
+    }
+
+	/**
+	 * Called after a successful retry attempt.
+	 * @param result the result of the callback
+	 * @param <T> the type of the result
+	 */
+    default <T> void onSuccess(T result) {
+    }
+
+	/**
+	 * Called every time a retry attempt fails.
+	 * @param exception the exception thrown by the callback
+	 */
+    default void onFailure(Exception exception) {
+    }
+
+	/**
+	 * Called once the retry policy is exhausted.
+	 * @param exception the last exception thrown by the callback
+	 */
+    default void onMaxAttempts(Exception exception) {
+    }
+
+}

spring-core/src/main/java/org/springframework/core/retry/RetryOperations.java

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+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.core.retry;
+
+import java.util.concurrent.Callable;
+
+import org.jspecify.annotations.Nullable;
+
+/**
+ * Main entry point to the core retry functionality. Defines a set of retryable operations.
+ *
+ * <p>Implemented by {@link RetryTemplate}. Not often used directly, but a useful
+ * option to enhance testability, as it can easily be mocked or stubbed.
+ *
+ * @author Mahmoud Ben Hassine
+ * @since 7.0
+ */
+public interface RetryOperations {
+
+	/**
+	 * Retry the given callback (according to the retry policy configured at the implementation level)
+	 * until it succeeds or eventually throw an exception if the retry policy is exhausted.
+	 * @param retryCallback the callback to call initially and retry if needed
+	 * @return the callback's result
+	 * @param <R> the type of the callback's result
+	 * @throws Exception if the retry policy is exhausted
+	 */
+    <R> @Nullable R execute(Callable<R> retryCallback) throws Exception;
+
+}

spring-core/src/main/java/org/springframework/core/retry/RetryPolicy.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.core.retry;
+
+/**
+ * Strategy interface to define how to calculate the maximin number of retry attempts.
+ *
+ * @author Mahmoud Ben Hassine
+ * @since 7.0
+ */
+public interface RetryPolicy {
+
+    /**
+     * Return the maximum number of retry attempts
+     *
+     * @return the maximum number of retry attempts
+     */
+    int getMaxAttempts();
+
+}

spring-core/src/main/java/org/springframework/core/retry/RetryTemplate.java

@@ -0,0 +1,134 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.core.retry;
+
+import java.time.Duration;
+import java.util.concurrent.Callable;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
+
+import org.springframework.core.retry.support.MaxAttemptsRetryPolicy;
+import org.springframework.core.retry.support.backoff.FixedBackOffPolicy;
+import org.springframework.core.retry.support.listener.CompositeRetryListener;
+import org.springframework.util.Assert;
+
+/**
+ * A basic implementation of {@link RetryOperations} that uses a
+ * {@link RetryPolicy} and a {@link BackOffPolicy} to retry a
+ * {@link Callable} piece of code. By default, the callback will be called
+ * 3 times (<code>MaxAttemptsRetryPolicy(3)<code/>) with a fixed backoff
+ * of 1 second (<code>FixedBackOffPolicy(Duration.ofSeconds(1))</code>).
+ *
+ * <p>It is also possible to register a {@link RetryListener} to intercept and inject code
+ * during key retry phases (before a retry attempt, after a retry attempt, etc.).
+ *
+ * <p>All retry operations performed by this class are logged at debug level,
+ * using "org.springframework.core.retry.RetryTemplate" as log category.
+ *
+ * @author Mahmoud Ben Hassine
+ * @since 7.0
+ * @see RetryOperations
+ * @see RetryPolicy
+ * @see BackOffPolicy
+ * @see RetryListener
+ */
+public class RetryTemplate implements RetryOperations {
+
+	protected final Log logger = LogFactory.getLog(getClass());
+
+    private RetryPolicy retryPolicy = new MaxAttemptsRetryPolicy(3);
+
+    private BackOffPolicy backOffPolicy = new FixedBackOffPolicy(Duration.ofSeconds(1));
+
+    private RetryListener retryListener = new RetryListener() {};
+
+	/**
+	 * Set the {@link RetryPolicy} to use. Defaults to <code>MaxAttemptsRetryPolicy(3)</code>.
+	 * @param retryPolicy the retry policy to use. Must not be <code>null</code>.
+	 */
+	public void setRetryPolicy(RetryPolicy retryPolicy) {
+		Assert.notNull(retryPolicy, "Retry policy must not be null");
+		this.retryPolicy = retryPolicy;
+	}
+
+	/**
+	 * Set the {@link BackOffPolicy} to use. Defaults to <code>FixedBackOffPolicy(Duration.ofSeconds(1))</code>.
+	 * @param backOffPolicy the backoff policy to use. Must not be <code>null</code>.
+	 */
+	public void setBackOffPolicy(BackOffPolicy backOffPolicy) {
+		Assert.notNull(backOffPolicy, "BackOff policy must not be null");
+		this.backOffPolicy = backOffPolicy;
+	}
+
+	/**
+	 * Set a {@link RetryListener} to use. Defaults to a <code>NoOp</code> implementation.
+	 * If multiple listeners are needed, use a {@link CompositeRetryListener}.
+	 * @param retryListener the retry listener to use. Must not be <code>null</code>.
+	 */
+	public void setRetryListener(RetryListener retryListener) {
+		Assert.notNull(retryListener, "Retry listener must not be null");
+		this.retryListener = retryListener;
+	}
+
+	/**
+	 * Call the retry callback according to the configured retry and backoff policies.
+	 * If the callback succeeds, its result is returned. Otherwise, the last exception will
+	 * be propagated to the caller.
+	 * @param retryCallback the callback to call initially and retry if needed
+	 * @return the result of the callback if any
+	 * @param <R> the type of the result
+	 * @throws Exception thrown if the retry policy is exhausted
+	 */
+    @Override
+    public <R> @Nullable R execute(Callable<R> retryCallback) throws Exception {
+		Assert.notNull(retryCallback, "Retry Callback must not be null");
+        int attempts = 0;
+        int maxAttempts = retryPolicy.getMaxAttempts();
+        while(attempts++ <= maxAttempts) {
+			if (logger.isDebugEnabled()) {
+				logger.debug("Retry attempt #" + attempts);
+			}
+			try {
+                retryListener.beforeRetry();
+                R result = retryCallback.call();
+                retryListener.onSuccess(result);
+				if (logger.isDebugEnabled()) {
+					logger.debug("Retry attempt #" + attempts + " succeeded");
+				}
+				return result;
+            }
+			catch (Exception e) {
+                retryListener.onFailure(e);
+                Duration duration = backOffPolicy.backOff();
+                Thread.sleep(duration.toMillis());
+				if (logger.isDebugEnabled()) {
+					logger.debug("Attempt #" + attempts + " failed, backing off for " + duration.toMillis() + "ms");
+				}
+				if (attempts >= maxAttempts) {
+					if (logger.isDebugEnabled()) {
+						logger.debug("Maximum retry attempts " + attempts + " exhausted, aborting execution");
+					}
+					retryListener.onMaxAttempts(e);
+                    throw e;
+                }
+            }
+        }
+        return null;
+    }
+
+}

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

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+/**
+ * Main package for the core retry functionality.
+ */
+@NullMarked
+package org.springframework.core.retry;
+
+import org.jspecify.annotations.NullMarked;

spring-core/src/main/java/org/springframework/core/retry/support/AlwaysRetryPolicy.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.core.retry.support;
+
+import org.springframework.core.retry.RetryPolicy;
+
+/**
+ * A {@link RetryPolicy} that signals to the caller to always retry the callback.
+ *
+ * @author Mahmoud Ben Hassine
+ * @since 7.0
+ */
+public class AlwaysRetryPolicy implements RetryPolicy {
+
+	@Override
+	public int getMaxAttempts() {
+		return Integer.MAX_VALUE;
+	}
+
+}