Supports CharSequence on SQL provider method return type

Fixes gh-1134

Author: kazuki43zoo

src/main/java/org/apache/ibatis/builder/annotation/ProviderSqlSource.java

@@ -63,17 +63,15 @@ public ProviderSqlSource(Configuration configuration, Object provider, Class<?>
       providerMethodName = (String) provider.getClass().getMethod("method").invoke(provider);
 
       for (Method m : this.providerType.getMethods()) {
-        if (providerMethodName.equals(m.getName())) {
-          if (m.getReturnType() == String.class) {
-            if (providerMethod != null){
-              throw new BuilderException("Error creating SqlSource for SqlProvider. Method '"
-                      + providerMethodName + "' is found multiple in SqlProvider '" + this.providerType.getName()
-                      + "'. Sql provider method can not overload.");
-            }
-            this.providerMethod = m;
-            this.providerMethodArgumentNames = new ParamNameResolver(configuration, m).getNames();
-            this.providerMethodParameterTypes = m.getParameterTypes();
+        if (providerMethodName.equals(m.getName()) && CharSequence.class.isAssignableFrom(m.getReturnType())) {
+          if (providerMethod != null){
+            throw new BuilderException("Error creating SqlSource for SqlProvider. Method '"
+                    + providerMethodName + "' is found multiple in SqlProvider '" + this.providerType.getName()
+                    + "'. Sql provider method can not overload.");
           }
+          this.providerMethod = m;
+          this.providerMethodArgumentNames = new ParamNameResolver(configuration, m).getNames();
+          this.providerMethodParameterTypes = m.getParameterTypes();
         }
       }
     } catch (BuilderException e) {
@@ -166,7 +164,8 @@ private String invokeProviderMethod(Object... args) throws Exception {
     if (!Modifier.isStatic(providerMethod.getModifiers())) {
       targetObject = providerType.newInstance();
     }
-    return (String) providerMethod.invoke(targetObject, args);
+    CharSequence sql = (CharSequence) providerMethod.invoke(targetObject, args);
+    return sql != null ? sql.toString() : null;
   }
 
   private String replacePlaceholder(String sql) {

src/test/java/org/apache/ibatis/submitted/sqlprovider/SqlProviderTest.java

@@ -587,22 +587,24 @@ public interface StaticMethodSqlProviderMapper {
     String oneArgumentAndProviderContext(Integer value);
 
     class SqlProvider {
+
       public static String noArgument() {
         return "SELECT 1 FROM INFORMATION_SCHEMA.SYSTEM_USERS";
       }
 
-      public static String oneArgument(Integer value) {
-        return "SELECT " + value + " FROM INFORMATION_SCHEMA.SYSTEM_USERS";
+      public static StringBuilder oneArgument(Integer value) {
+        return new StringBuilder().append("SELECT ").append(value)
+            .append(" FROM INFORMATION_SCHEMA.SYSTEM_USERS");
       }
 
-      public static String multipleArgument(@Param("value1") Integer value1,
+      public static CharSequence multipleArgument(@Param("value1") Integer value1,
           @Param("value2") Integer value2) {
         return "SELECT " + (value1 + value2) + " FROM INFORMATION_SCHEMA.SYSTEM_USERS";
       }
 
-      public static String onlyProviderContext(ProviderContext context) {
-        return "SELECT '" + context.getMapperMethod().getName()
-            + "' FROM INFORMATION_SCHEMA.SYSTEM_USERS";
+      public static CharSequence onlyProviderContext(ProviderContext context) {
+        return new StringBuilder().append("SELECT '").append(context.getMapperMethod().getName())
+            .append("' FROM INFORMATION_SCHEMA.SYSTEM_USERS");
       }
 
       public static String oneArgumentAndProviderContext(Integer value, ProviderContext context) {