bug修复及设计增强：相应ExportParameterVisitor实现类扩展相应的ParameterizedOutputVisitor实现类，以修正ExportParameterVisitor现有实现的功能bug并减少重复代码

src/main/java/com/alibaba/druid/sql/dialect/db2/visitor/DB2ExportParameterVisitor.java

@@ -28,58 +28,94 @@ import com.alibaba.druid.sql.ast.statement.SQLSelectItem;
 import com.alibaba.druid.sql.visitor.ExportParameterVisitor;
 import com.alibaba.druid.sql.visitor.ExportParameterVisitorUtils;
 
-public class DB2ExportParameterVisitor extends DB2ASTVisitorAdapter implements ExportParameterVisitor {
+public class DB2ExportParameterVisitor extends DB2ParameterizedOutputVisitor implements ExportParameterVisitor {
 
-    private final List<Object> parameters;
 
-    public DB2ExportParameterVisitor(){
+    /**
+     * true= if require parameterized sql output
+     */
+    private final boolean requireParameterizedOutput;
+
+    public DB2ExportParameterVisitor(final List<Object> parameters,final Appendable appender,final boolean wantParameterizedOutput){
+        super(appender);
+        this.parameters = parameters;
+        this.requireParameterizedOutput = wantParameterizedOutput;
+    }
+
+    public DB2ExportParameterVisitor() {
         this(new ArrayList<Object>());
     }
 
-    public DB2ExportParameterVisitor(List<Object> parameters){
-        this.parameters = parameters;
+    public DB2ExportParameterVisitor(final List<Object> parameters){
+        this(parameters,new StringBuilder(),false);
     }
 
+    public DB2ExportParameterVisitor(final Appendable appender) {
+        this(new ArrayList<Object>(),appender,true);
+    }
+
+    
     public List<Object> getParameters() {
         return parameters;
     }
 
     @Override
     public boolean visit(SQLSelectItem x) {
+        if(requireParameterizedOutput){
+            return super.visit(x);
+        }
         return false;
     }
 
     @Override
     public boolean visit(SQLOrderBy x) {
+        if(requireParameterizedOutput){
+            return super.visit(x);
+        }
         return false;
     }
 
     @Override
     public boolean visit(SQLSelectGroupByClause x) {
+        if(requireParameterizedOutput){
+            return super.visit(x);
+        }
         return false;
     }
 
     @Override
     public boolean visit(SQLMethodInvokeExpr x) {
+        if(requireParameterizedOutput){
+            return super.visit(x);
+        }
         ExportParameterVisitorUtils.exportParamterAndAccept(this.parameters, x.getParameters());
-
         return true;
     }
 
     @Override
     public boolean visit(SQLInListExpr x) {
+        if(requireParameterizedOutput){
+            return super.visit(x);
+        }
+        
         ExportParameterVisitorUtils.exportParamterAndAccept(this.parameters, x.getTargetList());
-
         return true;
     }
 
     @Override
     public boolean visit(SQLBetweenExpr x) {
+        if(requireParameterizedOutput){
+            return super.visit(x);
+        }
+
         ExportParameterVisitorUtils.exportParameter(this.parameters, x);
         return true;
     }
 
     public boolean visit(SQLBinaryOpExpr x) {
+        if(requireParameterizedOutput){
+            return super.visit(x);
+        }
         ExportParameterVisitorUtils.exportParameter(this.parameters, x);
         return true;
     }