feat: Added hook data

src/main/java/dev/openfeature/sdk/HookContext.java

@@ -11,7 +11,7 @@
  * @param <T> the type for the flag being evaluated
  */
 @Value
-@Builder
+@Builder(toBuilder = true)
 @With
 public class HookContext<T> {
     @NonNull String flagKey;
@@ -25,6 +25,12 @@ public class HookContext<T> {
     ClientMetadata clientMetadata;
     Metadata providerMetadata;
 
+    /**
+     * Hook data provides a way for hooks to maintain state across their execution stages.
+     * Each hook instance gets its own isolated data store.
+     */
+    HookData hookData;
+
     /**
      * Builds a {@link HookContext} instances from request data.
      *
@@ -51,6 +57,7 @@ public static <T> HookContext<T> from(
                 .providerMetadata(providerMetadata)
                 .ctx(ctx)
                 .defaultValue(defaultValue)
+                .hookData(null) // Explicitly set to null for backward compatibility
                 .build();
     }
 }

src/main/java/dev/openfeature/sdk/HookData.java

@@ -0,0 +1,75 @@
+package dev.openfeature.sdk;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Hook data provides a way for hooks to maintain state across their execution stages.
+ * Each hook instance gets its own isolated data store that persists only for the duration
+ * of a single flag evaluation.
+ */
+public interface HookData {
+
+    /**
+     * Sets a value for the given key.
+     *
+     * @param key the key to store the value under
+     * @param value the value to store
+     */
+    void set(String key, Object value);
+
+    /**
+     * Gets the value for the given key.
+     *
+     * @param key the key to retrieve the value for
+     * @return the value, or null if not found
+     */
+    Object get(String key);
+
+    /**
+     * Gets the value for the given key, cast to the specified type.
+     *
+     * @param <T> the type to cast to
+     * @param key the key to retrieve the value for
+     * @param type the class to cast to
+     * @return the value cast to the specified type, or null if not found
+     * @throws ClassCastException if the value cannot be cast to the specified type
+     */
+    <T> T get(String key, Class<T> type);
+
+    /**
+     * Default implementation is using a synchronized map. 
+     */
+    static HookData create() {
+        return new DefaultHookData();
+    }
+
+    /**
+     * Default thread-safe implementation of HookData.
+     */
+    public class DefaultHookData implements HookData {
+        private final Map<String, Object> data = new HashMap<>();
+
+        @Override
+        public void set(String key, Object value) {
+            data.put(key, value);
+        }
+
+        @Override
+        public Object get(String key) {
+            return data.get(key);
+        }
+
+        @Override
+        public <T> T get(String key, Class<T> type) {
+            Object value = data.get(key);
+            if (value == null) {
+                return null;
+            }
+            if (!type.isInstance(value)) {
+                throw new ClassCastException("Value for key '" + key + "' is not of type " + type.getName());
+            }
+            return type.cast(value);
+        }
+    }
+}

src/main/java/dev/openfeature/sdk/HookSupport.java

@@ -2,6 +2,7 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -87,10 +88,21 @@ private EvaluationContext callBeforeHooks(
         List<Hook> reversedHooks = new ArrayList<>(hooks);
         Collections.reverse(reversedHooks);
         EvaluationContext context = hookCtx.getCtx();
+
+        // Create hook data for each hook instance
+        Map<Hook, HookData> hookDataMap = new HashMap<>();
+        for (Hook hook : reversedHooks) {
+            if (hook.supportsFlagValueType(flagValueType)) {
+                hookDataMap.put(hook, HookData.create());
+            }
+        }
+
         for (Hook hook : reversedHooks) {
             if (hook.supportsFlagValueType(flagValueType)) {
-                Optional<EvaluationContext> optional =
-                        Optional.ofNullable(hook.before(hookCtx, hints)).orElse(Optional.empty());
+                // Create a new context with this hook's data
+                HookContext contextWithHookData = hookCtx.withHookData(hookDataMap.get(hook));
+                Optional<EvaluationContext> optional = Optional.ofNullable(hook.before(contextWithHookData, hints))
+                        .orElse(Optional.empty());
                 if (optional.isPresent()) {
                     context = context.merge(optional.get());
                 }

src/test/java/dev/openfeature/sdk/HookContextTest.java

@@ -1,6 +1,6 @@
 package dev.openfeature.sdk;
 
-import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.*;
 import static org.mockito.Mockito.mock;
 
 import org.junit.jupiter.api.Test;
@@ -29,4 +29,52 @@ void metadata_field_is_type_metadata() {
                     "The before stage MUST run before flag resolution occurs. It accepts a hook context (required) and hook hints (optional) as parameters. It has no return value.")
     @Test
     void not_applicable_for_dynamic_context() {}
+
+    @Test
+    void shouldCreateHookContextWithHookData() {
+        HookData hookData = HookData.create();
+        hookData.set("test", "value");
+
+        HookContext<String> context = HookContext.<String>builder()
+                .flagKey("test-flag")
+                .type(FlagValueType.STRING)
+                .defaultValue("default")
+                .ctx(new ImmutableContext())
+                .hookData(hookData)
+                .build();
+
+        assertNotNull(context.getHookData());
+        assertEquals("value", context.getHookData().get("test"));
+    }
+
+    @Test
+    void shouldCreateHookContextWithoutHookData() {
+        HookContext<String> context = HookContext.<String>builder()
+                .flagKey("test-flag")
+                .type(FlagValueType.STRING)
+                .defaultValue("default")
+                .ctx(new ImmutableContext())
+                .build();
+
+        assertNull(context.getHookData());
+    }
+
+    @Test
+    void shouldCreateHookContextWithHookDataUsingWith() {
+        HookContext<String> originalContext = HookContext.<String>builder()
+                .flagKey("test-flag")
+                .type(FlagValueType.STRING)
+                .defaultValue("default")
+                .ctx(new ImmutableContext())
+                .build();
+
+        HookData hookData = HookData.create();
+        hookData.set("timing", System.currentTimeMillis());
+
+        HookContext<String> contextWithHookData = originalContext.withHookData(hookData);
+
+        assertNull(originalContext.getHookData());
+        assertNotNull(contextWithHookData.getHookData());
+        assertNotNull(contextWithHookData.getHookData().get("timing"));
+    }
 }

src/test/java/dev/openfeature/sdk/HookDataTest.java

@@ -0,0 +1,111 @@
+package dev.openfeature.sdk;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import org.junit.jupiter.api.Test;
+
+class HookDataTest {
+
+    @Test
+    void shouldStoreAndRetrieveValues() {
+        HookData hookData = HookData.create();
+
+        hookData.set("key1", "value1");
+        hookData.set("key2", 42);
+        hookData.set("key3", true);
+
+        assertEquals("value1", hookData.get("key1"));
+        assertEquals(42, hookData.get("key2"));
+        assertEquals(true, hookData.get("key3"));
+    }
+
+    @Test
+    void shouldReturnNullForMissingKeys() {
+        HookData hookData = HookData.create();
+
+        assertNull(hookData.get("nonexistent"));
+    }
+
+    @Test
+    void shouldSupportTypeSafeRetrieval() {
+        HookData hookData = HookData.create();
+
+        hookData.set("string", "hello");
+        hookData.set("integer", 123);
+        hookData.set("boolean", false);
+
+        assertEquals("hello", hookData.get("string", String.class));
+        assertEquals(Integer.valueOf(123), hookData.get("integer", Integer.class));
+        assertEquals(Boolean.FALSE, hookData.get("boolean", Boolean.class));
+    }
+
+    @Test
+    void shouldReturnNullForMissingKeysWithType() {
+        HookData hookData = HookData.create();
+
+        assertNull(hookData.get("missing", String.class));
+    }
+
+    @Test
+    void shouldThrowClassCastExceptionForWrongType() {
+        HookData hookData = HookData.create();
+
+        hookData.set("string", "not a number");
+
+        assertThrows(ClassCastException.class, () -> {
+            hookData.get("string", Integer.class);
+        });
+    }
+
+    @Test
+    void shouldOverwriteExistingValues() {
+        HookData hookData = HookData.create();
+
+        hookData.set("key", "original");
+        assertEquals("original", hookData.get("key"));
+
+        hookData.set("key", "updated");
+        assertEquals("updated", hookData.get("key"));
+    }
+
+    @Test
+    void shouldBeThreadSafe() throws InterruptedException {
+        HookData hookData = HookData.create();
+        int threadCount = 10;
+        int operationsPerThread = 100;
+        CountDownLatch latch = new CountDownLatch(threadCount);
+        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+            final int threadId = i;
+            executor.submit(() -> {
+                try {
+                    for (int j = 0; j < operationsPerThread; j++) {
+                        String key = "thread-" + threadId + "-key-" + j;
+                        String value = "thread-" + threadId + "-value-" + j;
+                        hookData.set(key, value);
+                        assertEquals(value, hookData.get(key));
+                    }
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+
+        assertTrue(latch.await(10, TimeUnit.SECONDS));
+        executor.shutdown();
+    }
+
+    @Test
+    void shouldSupportNullValues() {
+        HookData hookData = HookData.create();
+
+        hookData.set("nullKey", null);
+        assertNull(hookData.get("nullKey"));
+        assertNull(hookData.get("nullKey", String.class));
+    }
+}

src/test/java/dev/openfeature/sdk/HookSupportTest.java

@@ -23,8 +23,14 @@ void shouldMergeEvaluationContextsOnBeforeHooksCorrectly() {
         Map<String, Value> attributes = new HashMap<>();
         attributes.put("baseKey", new Value("baseValue"));
         EvaluationContext baseContext = new ImmutableContext(attributes);
-        HookContext<String> hookContext = new HookContext<>(
-                "flagKey", FlagValueType.STRING, "defaultValue", baseContext, () -> "client", () -> "provider");
+        HookContext<String> hookContext = HookContext.<String>builder()
+                .flagKey("flagKey")
+                .type(FlagValueType.STRING)
+                .defaultValue("defaultValue")
+                .ctx(baseContext)
+                .clientMetadata(() -> "client")
+                .providerMetadata(() -> "provider")
+                .build();
         Hook<String> hook1 = mockStringHook();
         Hook<String> hook2 = mockStringHook();
         when(hook1.before(any(), any())).thenReturn(Optional.of(evaluationContextWithValue("bla", "blubber")));
@@ -47,13 +53,14 @@ void shouldAlwaysCallGenericHook(FlagValueType flagValueType) {
         HookSupport hookSupport = new HookSupport();
         EvaluationContext baseContext = new ImmutableContext();
         IllegalStateException expectedException = new IllegalStateException("All fine, just a test");
-        HookContext<Object> hookContext = new HookContext<>(
-                "flagKey",
-                flagValueType,
-                createDefaultValue(flagValueType),
-                baseContext,
-                () -> "client",
-                () -> "provider");
+        HookContext<Object> hookContext = HookContext.<Object>builder()
+                .flagKey("flagKey")
+                .type(flagValueType)
+                .defaultValue(createDefaultValue(flagValueType))
+                .ctx(baseContext)
+                .clientMetadata(() -> "client")
+                .providerMetadata(() -> "provider")
+                .build();
 
         hookSupport.beforeHooks(
                 flagValueType, hookContext, Collections.singletonList(genericHook), Collections.emptyMap());
@@ -105,4 +112,33 @@ private EvaluationContext evaluationContextWithValue(String key, String value) {
         EvaluationContext baseContext = new ImmutableContext(attributes);
         return baseContext;
     }
+
+    private static class TestHook implements Hook<String> {
+        boolean beforeCalled = false;
+        boolean afterCalled = false;
+        boolean errorCalled = false;
+        boolean finallyCalled = false;
+
+        @Override
+        public Optional<EvaluationContext> before(HookContext<String> ctx, Map<String, Object> hints) {
+            beforeCalled = true;
+            return Optional.empty();
+        }
+
+        @Override
+        public void after(HookContext<String> ctx, FlagEvaluationDetails<String> details, Map<String, Object> hints) {
+            afterCalled = true;
+        }
+
+        @Override
+        public void error(HookContext<String> ctx, Exception error, Map<String, Object> hints) {
+            errorCalled = true;
+        }
+
+        @Override
+        public void finallyAfter(
+                HookContext<String> ctx, FlagEvaluationDetails<String> details, Map<String, Object> hints) {
+            finallyCalled = true;
+        }
+    }
 }