Remove indirection and overhead from interceptors

client/client-core/src/main/java/software/amazon/smithy/java/client/core/Client.java

@@ -46,7 +46,7 @@ protected Client(Builder<?, ?> builder) {
         ClientConfig.Builder configBuilder = builder.configBuilder();
         this.config = configBuilder.build();
         this.pipeline = ClientPipeline.of(config.protocol(), config.transport());
-        this.interceptor = ClientInterceptor.chain(config.interceptors());
+        this.interceptor = config.interceptorChain();
         this.identityResolvers = IdentityResolvers.of(config.identityResolvers());
         this.typeRegistry = typeRegistry();
 
@@ -99,7 +99,7 @@ protected <I extends SerializableStruct, O extends SerializableStruct> O call(
         // Rebuild the pipeline, resolvers, etc if the config changed.
         if (callConfig != config) {
             callPipeline = ClientPipeline.of(callConfig.protocol(), callConfig.transport());
-            callInterceptor = ClientInterceptor.chain(callConfig.interceptors());
+            callInterceptor = callConfig.interceptorChain();
             callIdentityResolvers = IdentityResolvers.of(callConfig.identityResolvers());
         }
 

client/client-core/src/main/java/software/amazon/smithy/java/client/core/ClientConfig.java

@@ -44,6 +44,7 @@ public final class ClientConfig {
     private final ClientProtocol<?, ?> protocol;
     private final EndpointResolver endpointResolver;
     private final List<ClientInterceptor> interceptors;
+    private final ClientInterceptor interceptorChain;
     private final List<AuthScheme<?, ?>> supportedAuthSchemes;
     private final AuthSchemeResolver authSchemeResolver;
     private final List<IdentityResolver<?>> identityResolvers;
@@ -78,6 +79,7 @@ private ClientConfig(Builder builder) {
         }
 
         this.interceptors = List.copyOf(builder.interceptors);
+        this.interceptorChain = ClientInterceptor.chain(this.interceptors);
 
         // By default, support NoAuthAuthScheme
         List<AuthScheme<?, ?>> supportedAuthSchemes = new ArrayList<>();
@@ -178,6 +180,13 @@ public List<ClientInterceptor> interceptors() {
         return interceptors;
     }
 
+    /**
+     * @return A pre-built interceptor chain for the configured interceptors.
+     */
+    public ClientInterceptor interceptorChain() {
+        return interceptorChain;
+    }
+
     /**
      * @return Authentication schemes supported by the client.
      */

client/client-core/src/main/java/software/amazon/smithy/java/client/core/interceptors/ClientInterceptor.java

@@ -45,7 +45,11 @@ public interface ClientInterceptor {
      * @return the combined interceptors.
      */
     static ClientInterceptor chain(List<ClientInterceptor> interceptors) {
-        return interceptors.isEmpty() ? NOOP : new ClientInterceptorChain(interceptors);
+        return switch (interceptors.size()) {
+            case 0 -> NOOP;
+            case 1 -> interceptors.get(0);
+            default -> new ClientInterceptorChain(interceptors);
+        };
     }
 
     /**

client/client-core/src/main/java/software/amazon/smithy/java/client/core/interceptors/ClientInterceptorChain.java

@@ -6,37 +6,30 @@
 package software.amazon.smithy.java.client.core.interceptors;
 
 import java.util.List;
-import java.util.function.BiConsumer;
-import java.util.function.BiFunction;
 import software.amazon.smithy.java.client.core.ClientConfig;
 import software.amazon.smithy.java.core.schema.SerializableStruct;
 import software.amazon.smithy.java.logging.InternalLogger;
 
 final class ClientInterceptorChain implements ClientInterceptor {
 
     private static final InternalLogger LOGGER = InternalLogger.getLogger(ClientInterceptorChain.class);
-    private final List<ClientInterceptor> interceptors;
+    private final ClientInterceptor[] interceptors;
 
     public ClientInterceptorChain(List<ClientInterceptor> interceptors) {
         if (interceptors.isEmpty()) {
             throw new IllegalArgumentException("Interceptors cannot be empty");
         }
-        this.interceptors = interceptors;
+        this.interceptors = interceptors.toArray(ClientInterceptor[]::new);
     }
 
     @Override
     public void readBeforeExecution(InputHook<?, ?> hook) {
-        applyToEachThrowLastError("readBeforeExecution", ClientInterceptor::readBeforeExecution, hook);
-    }
-
-    // Many interceptors require running each hook, logging errors, and throwing the last.
-    private <T> void applyToEachThrowLastError(String hookName, BiConsumer<ClientInterceptor, T> consumer, T hook) {
         RuntimeException error = null;
         for (var interceptor : interceptors) {
             try {
-                consumer.accept(interceptor, hook);
+                interceptor.readBeforeExecution(hook);
             } catch (RuntimeException e) {
-                error = swapError(hookName, error, e);
+                error = swapError("readBeforeExecution", error, e);
             }
         }
 
@@ -79,28 +72,36 @@ public void readAfterSerialization(RequestHook<?, ?, ?> hook) {
 
     @Override
     public <RequestT> RequestT modifyBeforeRetryLoop(RequestHook<?, ?, RequestT> hook) {
-        return modifyRequestHook(ClientInterceptor::modifyBeforeRetryLoop, hook);
-    }
-
-    private <I extends SerializableStruct, RequestT> RequestT modifyRequestHook(
-            BiFunction<ClientInterceptor, RequestHook<I, ?, RequestT>, RequestT> mapper,
-            RequestHook<I, ?, RequestT> hook
-    ) {
         var request = hook.request();
         for (var interceptor : interceptors) {
-            request = mapper.apply(interceptor, hook.withRequest(request));
+            request = interceptor.modifyBeforeRetryLoop(hook.withRequest(request));
         }
         return request;
     }
 
     @Override
     public void readBeforeAttempt(RequestHook<?, ?, ?> hook) {
-        applyToEachThrowLastError("readBeforeAttempt", ClientInterceptor::readBeforeAttempt, hook);
+        RuntimeException error = null;
+        for (var interceptor : interceptors) {
+            try {
+                interceptor.readBeforeAttempt(hook);
+            } catch (RuntimeException e) {
+                error = swapError("readBeforeAttempt", error, e);
+            }
+        }
+
+        if (error != null) {
+            throw error;
+        }
     }
 
     @Override
     public <RequestT> RequestT modifyBeforeSigning(RequestHook<?, ?, RequestT> hook) {
-        return modifyRequestHook(ClientInterceptor::modifyBeforeSigning, hook);
+        var request = hook.request();
+        for (var interceptor : interceptors) {
+            request = interceptor.modifyBeforeSigning(hook.withRequest(request));
+        }
+        return request;
     }
 
     @Override
@@ -119,7 +120,11 @@ public void readAfterSigning(RequestHook<?, ?, ?> hook) {
 
     @Override
     public <RequestT> RequestT modifyBeforeTransmit(RequestHook<?, ?, RequestT> hook) {
-        return modifyRequestHook(ClientInterceptor::modifyBeforeTransmit, hook);
+        var request = hook.request();
+        for (var interceptor : interceptors) {
+            request = interceptor.modifyBeforeTransmit(hook.withRequest(request));
+        }
+        return request;
     }
 
     @Override