Add fast path for determining Codec

- Add `Codec.getMainClass()` for fast path, it allows users to extend `Codec` to change fast path
- Add fast path to default codecs
- Merge primitive type fast path into new fast path

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/codec/AbstractClassedCodec.java

@@ -36,5 +36,10 @@ public final boolean canDecode(MySqlReadableMetadata metadata, Class<?> target)
         return target.isAssignableFrom(this.type) && doCanDecode(metadata);
     }
 
+    @Override
+    public final Class<? extends T> getMainClass() {
+        return this.type;
+    }
+
     protected abstract boolean doCanDecode(MySqlReadableMetadata metadata);
 }

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/codec/AbstractPrimitiveCodec.java

@@ -41,11 +41,18 @@ abstract class AbstractPrimitiveCodec<T> implements PrimitiveCodec<T> {
 
     @Override
     public final boolean canDecode(MySqlReadableMetadata metadata, Class<?> target) {
-        return target.isAssignableFrom(boxedClass) && canPrimitiveDecode(metadata);
+        return (target.isAssignableFrom(boxedClass) || target.equals(primitiveClass)) && doCanDecode(metadata);
     }
 
     @Override
     public final Class<T> getPrimitiveClass() {
         return primitiveClass;
     }
+
+    @Override
+    public final Class<? extends T> getMainClass() {
+        return boxedClass;
+    }
+
+    protected abstract boolean doCanDecode(MySqlReadableMetadata metadata);
 }

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/codec/BlobCodec.java

@@ -49,6 +49,11 @@ final class BlobCodec implements MassiveCodec<Blob> {
     private BlobCodec() {
     }
 
+    @Override
+    public Class<? extends Blob> getMainClass() {
+        return Blob.class;
+    }
+
     @Override
     public Blob decode(ByteBuf value, MySqlReadableMetadata metadata, Class<?> target, boolean binary,
         CodecContext context) {

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/codec/BooleanCodec.java

@@ -52,7 +52,7 @@ public MySqlParameter encode(Object value, CodecContext context) {
     }
 
     @Override
-    public boolean canPrimitiveDecode(MySqlReadableMetadata metadata) {
+    public boolean doCanDecode(MySqlReadableMetadata metadata) {
         MySqlType type = metadata.getType();
         return (type == MySqlType.BIT || type == MySqlType.TINYINT) &&
             Integer.valueOf(1).equals(metadata.getPrecision());

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/codec/ByteCodec.java

@@ -52,7 +52,7 @@ public MySqlParameter encode(Object value, CodecContext context) {
     }
 
     @Override
-    public boolean canPrimitiveDecode(MySqlReadableMetadata metadata) {
+    public boolean doCanDecode(MySqlReadableMetadata metadata) {
         return metadata.getType().isNumeric();
     }
 

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/codec/ClobCodec.java

@@ -52,6 +52,11 @@ final class ClobCodec implements MassiveCodec<Clob> {
     private ClobCodec() {
     }
 
+    @Override
+    public Class<? extends Clob> getMainClass() {
+        return Clob.class;
+    }
+
     @Override
     public Clob decode(ByteBuf value, MySqlReadableMetadata metadata, Class<?> target, boolean binary,
         CodecContext context) {

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/codec/Codec.java

@@ -41,8 +41,7 @@ public interface Codec<T> {
      * @return the decoded result.
      */
     @Nullable
-    T decode(ByteBuf value, MySqlReadableMetadata metadata, Class<?> target, boolean binary,
-        CodecContext context);
+    T decode(ByteBuf value, MySqlReadableMetadata metadata, Class<?> target, boolean binary, CodecContext context);
 
     /**
      * Checks if the field value can be decoded as specified {@link Class}.
@@ -69,4 +68,16 @@ T decode(ByteBuf value, MySqlReadableMetadata metadata, Class<?> target, boolean
      * @return encoded {@link MySqlParameter}.
      */
     MySqlParameter encode(Object value, CodecContext context);
+
+    /**
+     * Gets the main {@link Class} that is handled by this codec. It is used to fast path the codec lookup if it is not
+     * {@code null}. If same main {@link Class} is handled by multiple codecs, the codec with the highest priority will
+     * be used. The priority of the fast path is determined by its order in {@link Codecs}.
+     *
+     * @return the main {@link Class}, or {@code null} if it is not in fast path.
+     */
+    @Nullable
+    default Class<? extends T> getMainClass() {
+        return null;
+    }
 }