fix: add read/write locks to client/api

Signed-off-by: Todd Baert <[email protected]>

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

@@ -1,43 +1,45 @@
 package dev.openfeature.sdk;
 
-import lombok.Getter;
-import lombok.Setter;
-
-import javax.annotation.Nullable;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
+import javax.annotation.Nullable;
+
+import dev.openfeature.sdk.internal.AutoCloseableLock;
+import dev.openfeature.sdk.internal.AutoCloseableReentrantReadWriteLock;
+import lombok.Getter;
+import lombok.Setter;
+
 /**
  * A global singleton which holds base configuration for the OpenFeature library.
  * Configuration here will be shared across all {@link Client}s.
  */
 public class OpenFeatureAPI {
-    private static OpenFeatureAPI api;
+    // package-private multi-read/single-write lock
+    static AutoCloseableReentrantReadWriteLock rwLock = new AutoCloseableReentrantReadWriteLock();
     @Getter
-    @Setter
     private FeatureProvider provider;
     @Getter
     @Setter
     private EvaluationContext evaluationContext;
     @Getter
     private List<Hook> apiHooks;
 
-    public OpenFeatureAPI() {
+    private OpenFeatureAPI() {
         this.apiHooks = new ArrayList<>();
     }
 
+    private static class SingletonHolder {
+        private static final OpenFeatureAPI INSTANCE = new OpenFeatureAPI();
+    }
+
     /**
      * Provisions the {@link OpenFeatureAPI} singleton (if needed) and returns it.
      * @return The singleton instance.
      */
     public static OpenFeatureAPI getInstance() {
-        synchronized (OpenFeatureAPI.class) {
-            if (api == null) {
-                api = new OpenFeatureAPI();
-            }
-        }
-        return api;
+        return SingletonHolder.INSTANCE;
     }
 
     public Metadata getProviderMetadata() {
@@ -56,11 +58,30 @@ public Client getClient(@Nullable String name, @Nullable String version) {
         return new OpenFeatureClient(this, name, version);
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public void setProvider(FeatureProvider provider) {
+        try (AutoCloseableLock __ = rwLock.writeLockAutoCloseable()) {
+            this.provider = provider;
+        }
+    }
+
+    /**
+     * {@inheritDoc}
+     */
     public void addHooks(Hook... hooks) {
-        this.apiHooks.addAll(Arrays.asList(hooks));
+        try (AutoCloseableLock __ = rwLock.writeLockAutoCloseable()) {
+            this.apiHooks.addAll(Arrays.asList(hooks));
+        }
     }
 
+    /**
+     * {@inheritDoc}
+     */
     public void clearHooks() {
-        this.apiHooks.clear();
+        try (AutoCloseableLock __ = rwLock.writeLockAutoCloseable()) {
+            this.apiHooks.clear();
+        }
     }
 }

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

@@ -8,6 +8,8 @@
 
 import dev.openfeature.sdk.exceptions.GeneralError;
 import dev.openfeature.sdk.exceptions.OpenFeatureError;
+import dev.openfeature.sdk.internal.AutoCloseableLock;
+import dev.openfeature.sdk.internal.AutoCloseableReentrantReadWriteLock;
 import dev.openfeature.sdk.internal.ObjectUtils;
 import lombok.Getter;
 import lombok.Setter;
@@ -25,6 +27,7 @@ public class OpenFeatureClient implements Client {
     @Getter
     private final List<Hook> clientHooks;
     private final HookSupport hookSupport;
+    private AutoCloseableReentrantReadWriteLock rwLock = new AutoCloseableReentrantReadWriteLock();
 
     @Getter
     @Setter
@@ -48,7 +51,9 @@ public OpenFeatureClient(OpenFeatureAPI openFeatureAPI, String name, String vers
 
     @Override
     public void addHooks(Hook... hooks) {
-        this.clientHooks.addAll(Arrays.asList(hooks));
+        try (AutoCloseableLock __ = this.rwLock.writeLockAutoCloseable()) {
+            this.clientHooks.addAll(Arrays.asList(hooks));
+        }
     }
 
     private <T> FlagEvaluationDetails<T> evaluateFlag(FlagValueType type, String key, T defaultValue,
@@ -57,16 +62,19 @@ private <T> FlagEvaluationDetails<T> evaluateFlag(FlagValueType type, String key
                 () -> FlagEvaluationOptions.builder().build());
         Map<String, Object> hints = Collections.unmodifiableMap(flagOptions.getHookHints());
         ctx = ObjectUtils.defaultIfNull(ctx, () -> new MutableContext());
-        FeatureProvider provider = ObjectUtils.defaultIfNull(openfeatureApi.getProvider(), () -> {
-            log.debug("No provider configured, using no-op provider.");
-            return new NoOpProvider();
-        });
+
 
         FlagEvaluationDetails<T> details = null;
         List<Hook> mergedHooks = null;
         HookContext<T> hookCtx = null;
 
-        try {
+        try (AutoCloseableLock __ = OpenFeatureAPI.rwLock.readLockAutoCloseable();
+            AutoCloseableLock ___ = this.rwLock.readLockAutoCloseable()) {
+
+            FeatureProvider provider = ObjectUtils.defaultIfNull(openfeatureApi.getProvider(), () -> {
+                log.debug("No provider configured, using no-op provider.");
+                return new NoOpProvider();
+            });
 
             hookCtx = HookContext.from(key, type, this.getMetadata(),
                     openfeatureApi.getProvider().getMetadata(), ctx, defaultValue);

src/main/java/dev/openfeature/sdk/internal/AutoCloseableLock.java

@@ -0,0 +1,10 @@
+package dev.openfeature.sdk.internal;
+
+public interface AutoCloseableLock extends AutoCloseable {
+
+    /**
+     * Override the exception in AutoClosable.
+     */
+    @Override
+    void close();
+}

src/main/java/dev/openfeature/sdk/internal/AutoCloseableReentrantReadWriteLock.java

@@ -0,0 +1,28 @@
+package dev.openfeature.sdk.internal;
+
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+/**
+ * A utility class that wraps a multi-read/single-write lock construct as AutoCloseable, so it can
+ * be used in a try-with-resources.
+ */
+public class AutoCloseableReentrantReadWriteLock extends ReentrantReadWriteLock {
+
+    /**
+     * Get the single write lock as an AutoCloseableLock.
+     * @return unlock method ref
+     */
+    public AutoCloseableLock writeLockAutoCloseable() {
+        this.writeLock().lock();
+        return this.writeLock()::unlock;
+    }
+
+    /**
+     * Get the multi read lock as an AutoCloseableLock.
+     * @return unlock method ref
+     */
+    public AutoCloseableLock readLockAutoCloseable() {
+        this.readLock().lock();
+        return this.readLock()::unlock;
+    }
+}