add mapping support

src/main/java/com/alibaba/druid/mapping/MappingEngine.java

@@ -0,0 +1,63 @@
+package com.alibaba.druid.mapping;
+
+import java.util.LinkedHashMap;
+
+import com.alibaba.druid.sql.SQLUtils;
+import com.alibaba.druid.sql.ast.expr.SQLNumberExpr;
+import com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlSelectQueryBlock;
+import com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlSelectQueryBlock.Limit;
+import com.alibaba.druid.sql.dialect.mysql.parser.MySqlSelectParser;
+
+public class MappingEngine {
+
+    private LinkedHashMap<String, Entity> entities = new LinkedHashMap<String, Entity>();
+    private Integer                       maxLimit;
+
+    public Integer getMaxLimit() {
+        return maxLimit;
+    }
+
+    public void setMaxLimit(Integer maxLimit) {
+        this.maxLimit = maxLimit;
+    }
+
+    public LinkedHashMap<String, Entity> getEntities() {
+        return entities;
+    }
+
+    public void addEntity(Entity entity) {
+        this.entities.put(entity.getName(), entity);
+    }
+
+    public EntityMapingMySqlVisitor createMappingVisitor() {
+        return new EntityMapingMySqlVisitor(entities);
+    }
+
+    public String explain(String sql) {
+        MySqlSelectQueryBlock query = explainToSQLObject(sql);
+
+        if (this.maxLimit != null) {
+            if (query.getLimit() == null) {
+                Limit limit = new Limit();
+                limit.setRowCount(new SQLNumberExpr(maxLimit));
+                query.setLimit(limit);
+            } else {
+                SQLNumberExpr rowCountExpr = (SQLNumberExpr) query.getLimit().getRowCount();
+                int rowCount = rowCountExpr.getNumber().intValue();
+                if (rowCount > maxLimit.intValue()) {
+                    rowCountExpr.setNumber(maxLimit);
+                }
+            }
+        }
+
+        query.accept(this.createMappingVisitor());
+
+        return SQLUtils.toMySqlString(query);
+    }
+
+    public MySqlSelectQueryBlock explainToSQLObject(String sql) {
+        MySqlSelectParser selectParser = new MySqlSelectParser(sql);
+        MySqlSelectQueryBlock query = (MySqlSelectQueryBlock) selectParser.query();
+        return query;
+    }
+}