[3.3] Introduce preferred-protocol for consumer and provider to automatically negotiate the protocol to consume.

dubbo-common/src/main/java/org/apache/dubbo/common/constants/CommonConstants.java

@@ -629,6 +629,8 @@ public interface CommonConstants {
 
     String EXT_PROTOCOL = "ext.protocol";
 
+    String PREFERRED_PROTOCOL = "preferred.protocol";
+
     String IS_EXTRA = "isExtra";
 
     String ZOOKEEPER_ENSEMBLE_TRACKER_KEY = "zookeeper.ensemble.tracker";

dubbo-common/src/main/java/org/apache/dubbo/config/ProtocolConfig.java

@@ -231,6 +231,8 @@ public class ProtocolConfig extends AbstractConfig {
      */
     private String extProtocol;
 
+    private String preferredProtocol;
+
     /**
      * JSON check level for serialization.
      */
@@ -629,6 +631,14 @@ public void setExtProtocol(String extProtocol) {
         this.extProtocol = extProtocol;
     }
 
+    public String getPreferredProtocol() {
+        return preferredProtocol;
+    }
+
+    public void setPreferredProtocol(String preferredProtocol) {
+        this.preferredProtocol = preferredProtocol;
+    }
+
     public Boolean isNoInterfaceSupport() {
         return noInterfaceSupport;
     }

dubbo-common/src/main/java/org/apache/dubbo/config/context/AbstractConfigManager.java

@@ -558,7 +558,7 @@ public <T extends AbstractConfig> List<T> loadConfigsOfTypeFromProps(Class<T> cl
                     config.refresh();
                 } catch (Exception e) {
                     throw new IllegalStateException(
-                            "create default config instance failed, type:" + cls.getSimpleName());
+                            "create default config instance failed, type:" + cls.getSimpleName(), e);
                 }
 
                 this.addConfig(config);

dubbo-config/dubbo-config-spring/src/main/resources/META-INF/compat/dubbo.xsd

@@ -1305,6 +1305,11 @@
                 <xsd:documentation><![CDATA[ Is SSL enabled. ]]></xsd:documentation>
             </xsd:annotation>
         </xsd:attribute>
+        <xsd:attribute name="preferred-protocol" type="xsd:string">
+            <xsd:annotation>
+                <xsd:documentation><![CDATA[ Tell consumer the preferred protocol to consume. ]]></xsd:documentation>
+            </xsd:annotation>
+        </xsd:attribute>
         <xsd:anyAttribute namespace="##other" processContents="lax"/>
     </xsd:complexType>
 

dubbo-config/dubbo-config-spring/src/main/resources/META-INF/dubbo.xsd

@@ -1791,6 +1791,11 @@
                 <xsd:documentation><![CDATA[ Is SSL enabled. ]]></xsd:documentation>
             </xsd:annotation>
         </xsd:attribute>
+        <xsd:attribute name="preferred-protocol" type="xsd:string">
+            <xsd:annotation>
+                <xsd:documentation><![CDATA[ Tell consumer the preferred protocol to consume. ]]></xsd:documentation>
+            </xsd:annotation>
+        </xsd:attribute>
         <xsd:anyAttribute namespace="##other" processContents="lax"/>
     </xsd:complexType>
 

dubbo-registry/dubbo-registry-api/src/main/java/org/apache/dubbo/registry/integration/RegistryDirectory.java

@@ -68,6 +68,7 @@
 import static org.apache.dubbo.common.constants.CommonConstants.EXT_PROTOCOL;
 import static org.apache.dubbo.common.constants.CommonConstants.GROUP_KEY;
 import static org.apache.dubbo.common.constants.CommonConstants.INTERFACE_REGISTER_MODE;
+import static org.apache.dubbo.common.constants.CommonConstants.PREFERRED_PROTOCOL;
 import static org.apache.dubbo.common.constants.CommonConstants.PROTOCOL_KEY;
 import static org.apache.dubbo.common.constants.CommonConstants.SIDE_KEY;
 import static org.apache.dubbo.common.constants.CommonConstants.VERSION_KEY;
@@ -508,31 +509,40 @@ private Map<URL, Invoker<T>> toInvokers(Map<URL, Invoker<T>> oldUrlInvokerMap, L
     /**
      * Get the protocol to consume by matching the consumer acceptable protocols and the available provider protocols.
      * <p>
-     * Only one protocol will be used if consumer set to accept multiple protocols, for example, dubbo.consumer.protocol='tri,rest'.
+     * Only the first protocol that can match with the provider protocols will be used if consumer set to accept multiple protocols.
+     * For example, if dubbo.consumer.protocol='tri,rest' is set and provider provides tri protocol, then consumer will use tri protocol to communicate with provider.
      *
      * @param queryProtocols consumer side protocols.
      * @param url            provider url that have extra protocols specified.
      * @return the protocol to consume.
      */
     private String getEffectiveProtocol(String queryProtocols, URL url) {
         String protocol = url.getProtocol();
+        String prioritizedProtocol = url.getParameter(PREFERRED_PROTOCOL, protocol);
+
+        String effectiveProtocol = prioritizedProtocol;
+
         if (StringUtils.isNotEmpty(queryProtocols)) {
             String[] acceptProtocols = queryProtocols.split(COMMA_SEPARATOR);
             String acceptedProtocol = acceptProtocols[0];
-            if (!acceptedProtocol.equals(url.getProtocol())) {
-                String extProtocols = url.getParameter(EXT_PROTOCOL);
-                if (StringUtils.isNotEmpty(extProtocols)) {
-                    String[] extProtocolsArr = extProtocols.split(COMMA_SEPARATOR);
-                    for (String p : extProtocolsArr) {
-                        if (p.equalsIgnoreCase(acceptedProtocol)) {
-                            protocol = acceptedProtocol;
-                            break;
+            if (!acceptedProtocol.equals(prioritizedProtocol)) {
+                if (!acceptedProtocol.equals(protocol)) {
+                    String extProtocols = url.getParameter(EXT_PROTOCOL);
+                    if (StringUtils.isNotEmpty(extProtocols)) {
+                        String[] extProtocolsArr = extProtocols.split(COMMA_SEPARATOR);
+                        for (String p : extProtocolsArr) {
+                            if (p.equalsIgnoreCase(acceptedProtocol)) {
+                                effectiveProtocol = acceptedProtocol;
+                                break;
+                            }
                         }
                     }
+                } else {
+                    effectiveProtocol = protocol;
                 }
             }
         }
-        return protocol;
+        return effectiveProtocol;
     }
 
     private boolean checkProtocolValid(String queryProtocols, URL providerUrl) {