构建动态sql，其实说白了就是拼装sql语句，在这里我把传入的实体参数，属性有值的拼装进sql，为null的则忽略，要实现这个不用说，肯定要利用Java的反射功能，来看一个具有代表性的insert语句的构建：
public static SqlContext buildInsertSql(Object entity, NameHandler nameHandler) { Class clazz = entity.getClass(); String tableName = nameHandler.getTableName(clazz.getSimpleName()); String primaryName = nameHandler.getPrimaryName(clazz.getSimpleName()); StringBuilder sql = new StringBuilder( "insert into " ); List params = new ArrayList(); sql.append(tableName); BeanInfo beanInfo = ClassUtils.getSelfBeanInfo(clazz); PropertyDescriptor[] pds = beanInfo.getPropertyDescriptors(); sql.append("(" ); StringBuilder args = new StringBuilder(); args.append("(" ); for (PropertyDescriptor pd : pds) { Object value = getReadMethodValue(pd.getReadMethod(), entity); if (value == null ) { continue ; } sql.append(nameHandler.getColumnName(pd.getName())); args.append("?" ); params.add(value); sql.append("," ); args.append("," ); } sql.deleteCharAt(sql.length() - 1 ); args.deleteCharAt(args.length() - 1 ); args.append(")" ); sql.append(")" ); sql.append(" values " ); sql.append(args); return new SqlContext(sql, primaryName, params); }
众所周知，Java的反射是性能较低的，也有性能较好的第三方实现如cglib，这里并没有使用。在我的实测中两者差距不大。
但是注意这里并没有使用属性的操作方式，也就是没有使用jdk反射获取属性的getDeclaredFields()方法，而是使用了BeanInfo和PropertyDescriptor，因为后者的运行效率要远远高于前者。
在我的实测中，构建一个拥有12个属性的JavaBean的动态sql，十万次所耗时间为900毫秒左右，完全可以接受。当然，这里对JavaBean的信息进行了缓存，如果不缓存时间将多耗上几个数量级。
下面顺便贴上完整的代码：
public class SqlUtils { private static final Logger LOG = LoggerFactory.getLogger(SqlUtils. class ); public static SqlContext buildInsertSql(Object entity, NameHandler nameHandler) { Class clazz = entity.getClass(); String tableName = nameHandler.getTableName(clazz.getSimpleName()); String primaryName = nameHandler.getPrimaryName(clazz.getSimpleName()); StringBuilder sql = new StringBuilder( "insert into " ); List params = new ArrayList(); sql.append(tableName); BeanInfo beanInfo = ClassUtils.getSelfBeanInfo(clazz); PropertyDescriptor[] pds = beanInfo.getPropertyDescriptors(); sql.append("(" ); StringBuilder args = new StringBuilder(); args.append("(" ); for (PropertyDescriptor pd : pds) { Object value = getReadMethodValue(pd.getReadMethod(), entity); if (value == null ) { continue ; } sql.append(nameHandler.getColumnName(pd.getName())); args.append("?" ); params.add(value); sql.append("," ); args.append("," ); } sql.deleteCharAt(sql.length() - 1 ); args.deleteCharAt(args.length() - 1 ); args.append(")" ); sql.append(")" ); sql.append(" values " ); sql.append(args); return new SqlContext(sql, primaryName, params); } public static SqlContext buildUpdateSql(Object entity, NameHandler nameHandler) { Class clazz = entity.getClass(); StringBuilder sql = new StringBuilder(); List params = new ArrayList(); String tableName = nameHandler.getTableName(clazz.getSimpleName()); String primaryName = nameHandler.getPrimaryName(clazz.getSimpleName()); BeanInfo beanInfo = ClassUtils.getSelfBeanInfo(clazz); PropertyDescriptor[] pds = beanInfo.getPropertyDescriptors(); sql.append("update " ); sql.append(tableName); sql.append(" set " ); Object primaryValue = null ; for (PropertyDescriptor pd : pds) { Object value = getReadMethodValue(pd.getReadMethod(), entity); if (value == null ) { continue ; } String columnName = nameHandler.getColumnName(pd.getName()); if (primaryName.equalsIgnoreCase(columnName)) { primaryValue = value; } sql.append(columnName); sql.append(" = " ); sql.append("?" ); params.add(value); sql.append("," ); } sql.deleteCharAt(sql.length() - 1 ); sql.append(" where " ); sql.append(primaryName); sql.append(" = ?" ); params.add(primaryValue); return new SqlContext(sql, primaryName, params); } public static SqlContext buildQueryCondition(Object entity, NameHandler nameHandler) { BeanInfo beanInfo = ClassUtils.getSelfBeanInfo(entity.getClass()); PropertyDescriptor[] pds = beanInfo.getPropertyDescriptors(); StringBuilder condition = new StringBuilder(); List params = new ArrayList(); int count = 0 ; for (PropertyDescriptor pd : pds) { Object value = getReadMethodValue(pd.getReadMethod(), entity); if (value == null ) { continue ; } if (count > 0 ) { condition.append(" and " ); } condition.append(nameHandler.getColumnName(pd.getName())); condition.append(" = ?" ); params.add(value); count++; } return new SqlContext(condition, null , params); } private static Object getReadMethodValue(Method readMethod, Object entity) { if (readMethod == null ) { return null ; } try { if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers())) { readMethod.setAccessible(true ); } return readMethod.invoke(entity); } catch (Exception e) { LOG.error("获取属性值失败" , e); throw new MincoderException(e); } } }
获取BeanInfo时写了一个ClassUtils来实现，里面对Bean信息进行了缓存。因为项目使用spring，本来想使用spring提供的BeanUtils.getPropertyDescriptor()方法的，里面同样拥有缓存，但是该方法会把实体类父类的属性信息也获取出来，而PropertyDescriptor中又没法判断，这将直接导致拼装sql时字段的错误，因为你不知道哪些字段是操作当前表所需要的。没办法，查看jdk本身的Introspector类，发现里面有如下方法定义：
public static BeanInfo getBeanInfo(Class beanClass, Class stopClass) throws IntrospectionException
即可以指定在哪个类停止获取属性，这正是我们需要的，可惜spring没有进行封装，只能自己实现了，参考了spring的实现，使用WeakHashMap来防止内存的溢出，及时清空Introspector本身的缓存：
public class ClassUtils {
private static final Logger LOG = LoggerFactory
.getLogger(ClassUtils.class );
private static final Map classCache = Collections
.synchronizedMap(new WeakHashMap());
public static BeanInfo getSelfBeanInfo(Class clazz) {
try {
BeanInfo beanInfo;
if (classCache.get(clazz) == null ) {
beanInfo = Introspector.getBeanInfo(clazz, clazz.getSuperclass());
classCache.put(clazz, beanInfo);
Class classToFlush = clazz;
do {
Introspector.flushFromCaches(classToFlush);
classToFlush = classToFlush.getSuperclass();
} while (classToFlush != null );
} else {
beanInfo = classCache.get(clazz);
}
return beanInfo;
} catch (IntrospectionException e) {
LOG.error("获取BeanInfo失败" , e);
throw new MincoderException(e);
}
}
public static Object newInstance(Class clazz) {
try {
return clazz.newInstance();
} catch (Exception e) {
LOG.error("根据class创建实例失败" , e);
throw new MincoderException(e);
}
}
}
另外创建了对象SqlContext来保存构建后的sql和参数信息，定义如下：
public class SqlContext {
private StringBuilder sql;
private String primaryKey;
private List params;
public SqlContext(StringBuilder sql, String primaryKey, List params) {
this .sql = sql;
this .primaryKey = primaryKey;
this .params = params;
}
}
