实现一个简单的OGNL解析器

介绍

对象导航图语言（Object Graph Navigation Language），简称OGNL，它是一种功能强大的表达式语言，经过它简单一致的表达式语法，能够存取对象的任意属性，调用对象的方法，遍历整个对象的结构图，实现字段类型转化等功能。它使用统一的表达式去存取对象的属性。这样能够更方便直观的取得数据。

例如如下对象结构：

订单Order类包含订单号orderId、用户User类对象, 用户User类包含用户姓名name、用户年龄age、地址类Address数组对象，地址类对象Address包含省province、市city、地址明细detail。

| Order
|-- orderId
|-- User  
|---- name
|---- age
|---- Address List 
|------ province 
|------ city  
|------ detail

使用方法

我们通过如下方式来访问不同的属性：

- 访问订单号：order.orderNo   
- 访问用户名：order.user.name  
- 访问用户年龄：order.user.age  
- 访问第一个地址所在省份：order.user.addressList[0].province    
- 访问第2个地址所在市：   order.user.addressList[1].city  
- 访问第2个地址的明细：   order.user.addressList[1].detail

如果上述Order、User、Address对象之间的组合结构用Map对象来存储，那么OGNL访问的表达式也一样，所以说OGNL是一种统一的表达式语言。

代码实现

在java中如何自己来实现一个简单的ognl解析器呢，在不知道对象类型的情况下，我们根据属性名访问对象的属性，肯定要用反射，话不说直接上代码：

如下代码有3类解析方式：不同类型的解析方式详见代码注解

普通对象：通过反射获取属性值以及属性class，属性class用于继续通过反射获取点号后面的属性值（如果有），依次类推不断迭代直到取出最后一层属性；
Map对象：map最简单直接通过key方式获取属性值；
数组或List：数组要注意基本类型的数组不能直接强制转换为Object[]，八种基本类型要分别单独处理。

java">import java.lang.reflect.Field;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * OGNL解析器
 * 对象导航图语言（Object Graph Navigation Language）
 * @author zhouyong
 * @date 2023/11/26 8:07 下午
 */
public class OgnlParser {

    /**
     * 根据属性名获取对象的属性值(支持OGNL表达式)
     * eg: order.user.age , user.age , user.addressList[0] , user.addressArray[0] , user.addressList[0].city
     *
     * @param obj 对象，可以是Map，也支持对象里面嵌套Map，或者Map里面嵌套对象，且支持多层嵌套
     * @param propertyNamePath
     * 属性名的全路径，多层对象嵌套属性支持链式操作、数组下标的访问方式
     *   eg1: "a" 返回属性a的值
     *   eg2: "a[0]" 属性a为数组或者List，返回a的第一个元素值
     *   eg3: "a.b.c" 多层嵌套属性的访问方式，对象a包含对象b，对象b又包含c，通过该方式直接访问属性c的值
     *   eg4: "a.b.c[0]" 属性c为数组或者List，返回c的第一个元素值
     *   eg4: "a.b.c[0].d" 属性c为数组或者List，返回c的第一个元素的d属性值
     *
     * @return
     */
    public static <T> T getFieldValue(Object obj, String propertyNamePath){

        if(obj==null || propertyNamePath==null){
            return null;
        }

        T fieldValue = null;
        Object iteratorObj = obj;

        String[] fieldNames = propertyNamePath.split("\\.");
        List<String> fieldNameList = Stream.of(fieldNames).collect(Collectors.toList());
        Iterator<String> fieldNameIterator = fieldNameList.iterator();

        while(fieldNameIterator.hasNext()){
            String fieldName = fieldNameIterator.next();
            fieldValue = getPropertyValue(iteratorObj, fieldName);
            if(fieldValue!=null){
                if(fieldNameIterator.hasNext()){
                    iteratorObj = fieldValue;
                }
            }
        }

        return fieldValue;
    }

    /**
     * 根据属性名获取属性值
     * @param obj
     * @param propertyName
     * @return
     */
    private static <T> T getPropertyValue(Object obj, String propertyName){
        if(obj==null || propertyName==null){
            return null;
        }

        Class objClass = obj.getClass();

        T value = null;
        String fieldName = parseFieldName(propertyName);

        //如果是Map则直接通过key获取值
        if(Map.class.isAssignableFrom(objClass)){
            Object valueFromMap = (T)((Map) obj).get(fieldName);
            value = getElementIfArray(valueFromMap, parseIndex(propertyName));
        }
        //否则通过反射获取字段值
        else{
            Field field = getField(objClass, fieldName);
            if(field!=null){
                boolean accessible = field.isAccessible();
                if(!accessible){
                    field.setAccessible(true);
                }
                try {
                    Object valueFromField = field.get(obj);
                    value = getElementIfArray(valueFromField, parseIndex(propertyName));
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    if(!accessible){
                        field.setAccessible(false);
                    }
                }
            }
        }

        return value;
    }

    /**
     * 数组或List的特殊处理
     * @param value
     * @param index 不为空表示属性名带下标，index为对应的下标志，如： list[0]解析出的index为0
     * @return
     */
    private static <T> T getElementIfArray(Object value, Integer index) {
        if(value!=null && index!=null){
            Class valueType = value.getClass();

            //如果是List, 包含下标则返回对应的元素值
            if(List.class.isAssignableFrom(valueType)){
                List list = (List) value;
                if(index<list.size()){
                    value = list.get(index);
                }
            }

            //如果是数组，则返回指定下标的元素值
            else if(valueType.isArray()){
                //8种基本类型需要单独转换为对应基本类型的数组，直接转为Object[]会抛强制转换异常
                if(char[].class.equals(valueType)){
                    char[] array = (char[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }else if(byte[].class.equals(valueType)){
                    byte[] array = (byte[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }else if(short[].class.equals(valueType)){
                    short[] array = (short[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }else if(int[].class.equals(valueType)){
                    int[] array = (int[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }else if(long[].class.equals(valueType)){
                    long[] array = (long[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }else if(float[].class.equals(valueType)){
                    float[] array = (float[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }else if(double[].class.equals(valueType)){
                    double[] array = (double[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }else if(boolean[].class.equals(valueType)){
                    boolean[] array = (boolean[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }
                //非基本类型的数组可以直接强制转换为Object[]
                else{
                    Object[] array = (Object[]) value;
                    if(index<array.length){
                        value = array[index];
                    }
                }

            }

        }
        return (T) value;
    }

    /**
     * 根据字段名获取字段
     * @param objClass
     * @param fieldName
     * @return
     */
    private static Field getField(Class objClass, String fieldName){
        if(objClass==null || fieldName==null){
            return null;
        }
        Class superClass = objClass;
        Field field = null;
        do{
            try {
                field = superClass.getDeclaredField(fieldName);
            } catch (Exception e) {
                e.printStackTrace();
            }
            superClass = superClass.getSuperclass();
        }while(!Object.class.equals(superClass) && field==null);

        return field;
    }

    /**
     * 如果属性名带下标，则去掉下标解析出属性名
     * @param propertyName
     * @return
     */
    private static String parseFieldName(String propertyName) {
        int index = propertyName.indexOf("[");
        if(index>0){
            propertyName = propertyName.substring(0,index);

        }
        return propertyName;
    }

    /**
     * 如果属性名带下标，则解析下标
     * @param propertyName
     * @return
     */
    private static Integer parseIndex(String propertyName) {
        Integer index = null;
        int startIdx = propertyName.indexOf("[");
        if(startIdx>0 && propertyName.endsWith("]")){
            try {
                index = Integer.valueOf(propertyName.substring(startIdx+1, propertyName.length() - 1));
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return index;
    }

}

单元测试

针对上述代码我们在写一个单元测试验证：

java">/**
 * @author zhouyong
 * @date 2023/11/26 8:07 下午
 */
public class OgnlParserTest {

    @Test
    public void testByMap(){
        Map order = new HashMap(5);
        Map user = new HashMap(5);
        Map address = new HashMap(5);

        order.put("orderId", "order001");
        order.put("user", user);
        order.put("all", new Map[]{user,address});

        user.put("name","张三");
        user.put("sex", "男");
        user.put("numbers", new int[]{1,2,3});
        user.put("address", address);

        address.put("province", "广东省");
        address.put("city", "深圳市");

        Assert.assertEquals("order001", OgnlParser.getFieldValue(order, "orderId"));
        Assert.assertEquals(1, (int) OgnlParser.getFieldValue(order, "user.numbers[0]"));
        Assert.assertEquals("广东省", OgnlParser.getFieldValue(order, "user.address.province"));

        Assert.assertEquals("张三", OgnlParser.getFieldValue(order, "user.name"));
        Assert.assertEquals("张三", OgnlParser.getFieldValue(order, "all[0].name"));
        Assert.assertEquals("张三", OgnlParser.getFieldValue(user, "name"));
    }

    @Test
    public void testByObject(){
        Address address1 = new Address("广东省","深圳市", "罗湖区菜围街道");
        Address address2 = new Address("湖南省","长沙市", "岳麓区梅溪湖街道");
        List<Address> addressList = new ArrayList<>();
        addressList.add(address1);
        addressList.add(address2);

        User user = new User("user001", "张三", addressList, new String[]{"zhangsan@qq.com","zhangsan@163.com"});

        Map otherInfo = new HashMap(4);
        otherInfo.put("status", "已发货");
        otherInfo.put("user001", user);
        otherInfo.put("addressList", addressList);
        otherInfo.put("emails", user.emails);
        Order order = new Order("order001", new Date(), user, otherInfo);

        Assert.assertEquals(order.orderId, OgnlParser.getFieldValue(order, "orderId"));
        Assert.assertEquals(order.createdDate, OgnlParser.getFieldValue(order, "createdDate"));
        Assert.assertEquals(order.otherInfo.get("status"), OgnlParser.getFieldValue(order, "otherInfo.status"));

        Assert.assertEquals(order.users, OgnlParser.getFieldValue(order, "users"));
        Assert.assertEquals(order.users[0], OgnlParser.getFieldValue(order, "users[0]"));

        Assert.assertEquals(order.sequences, OgnlParser.getFieldValue(order, "sequences"));
        int sequence0 = OgnlParser.getFieldValue(order, "sequences[0]");
        Assert.assertEquals(order.sequences[0], sequence0);

        Assert.assertEquals(user.userId, OgnlParser.getFieldValue(order, "otherInfo.user001.userId"));
        Assert.assertEquals(user.userName, OgnlParser.getFieldValue(order, "otherInfo.user001.userName"));

        String[] emails = OgnlParser.getFieldValue(order, "otherInfo.emails");
        Assert.assertEquals(user.emails, emails);
        String emails0 = OgnlParser.getFieldValue(order, "otherInfo.emails[0]");
        Assert.assertEquals(user.emails[0], emails0);
        Assert.assertEquals(user.emails[1], OgnlParser.getFieldValue(order, "otherInfo.emails[1]"));

        Assert.assertEquals(user.emails, OgnlParser.getFieldValue(order, "otherInfo.user001.emails"));
        Assert.assertEquals(user.emails[0], OgnlParser.getFieldValue(order, "otherInfo.user001.emails[0]"));
        Assert.assertEquals(user.emails[1], OgnlParser.getFieldValue(order, "otherInfo.user001.emails[1]"));

        Assert.assertEquals(user.addressList, OgnlParser.getFieldValue(order, "otherInfo.addressList"));
        Assert.assertEquals(user.addressList.get(0), OgnlParser.getFieldValue(order, "otherInfo.addressList[0]"));
        Assert.assertEquals(user.addressList.get(1), OgnlParser.getFieldValue(order, "otherInfo.addressList[1]"));
        Assert.assertEquals(user.addressList.get(1).city, OgnlParser.getFieldValue(order, "otherInfo.addressList[1].city"));
        Assert.assertEquals(user.addressList.get(1).province, OgnlParser.getFieldValue(order, "otherInfo.addressList[1].province"));
        Assert.assertEquals(user.addressList.get(1).detail, OgnlParser.getFieldValue(order, "otherInfo.addressList[1].detail"));

        Assert.assertEquals(user.addressList, OgnlParser.getFieldValue(order, "otherInfo.user001.addressList"));
        Assert.assertEquals(user.addressList.get(0), OgnlParser.getFieldValue(order, "otherInfo.user001.addressList[0]"));
        Assert.assertEquals(user.addressList.get(1), OgnlParser.getFieldValue(order, "otherInfo.user001.addressList[1]"));
        Assert.assertEquals(user.addressList.get(1).city, OgnlParser.getFieldValue(order, "otherInfo.user001.addressList[1].city"));
        Assert.assertEquals(user.addressList.get(1).province, OgnlParser.getFieldValue(order, "otherInfo.user001.addressList[1].province"));
        Assert.assertEquals(user.addressList.get(1).detail, OgnlParser.getFieldValue(order, "otherInfo.user001.addressList[1].detail"));

        Assert.assertEquals(order.user.userId, OgnlParser.getFieldValue(order, "user.userId"));
        Assert.assertEquals(order.user.userName, OgnlParser.getFieldValue(order, "user.userName"));

        Assert.assertEquals(user.emails, OgnlParser.getFieldValue(order, "user.emails"));
        Assert.assertEquals(user.emails[0], OgnlParser.getFieldValue(order, "user.emails[0]"));
        Assert.assertEquals(user.emails[1], OgnlParser.getFieldValue(order, "user.emails[1]"));

        Assert.assertEquals(user.addressList, OgnlParser.getFieldValue(order, "user.addressList"));
        Assert.assertEquals(user.addressList.get(0), OgnlParser.getFieldValue(order, "user.addressList[0]"));
        Assert.assertEquals(user.addressList.get(1), OgnlParser.getFieldValue(order, "user.addressList[1]"));
        Assert.assertEquals(user.addressList.get(1).city, OgnlParser.getFieldValue(order, "user.addressList[1].city"));
        Assert.assertEquals(user.addressList.get(1).province, OgnlParser.getFieldValue(order, "user.addressList[1].province"));
        Assert.assertEquals(user.addressList.get(1).detail, OgnlParser.getFieldValue(order, "user.addressList[1].detail"));
    }

    class Order{
        private String orderId;
        private Date createdDate;
        private User user;
        private User[] users;
        private Map otherInfo;
        private int[] sequences = new int[]{1,2,3};

        public Order(String orderId, Date createdDate, User user, Map otherInfo) {
            this.orderId = orderId;
            this.createdDate = createdDate;
            this.user = user;
            this.otherInfo = otherInfo;
            this.users = new User[]{user};
        }
    }

    class User{
        private String userId;
        private String userName;
        private List<Address> addressList;
        private String[] emails;

        public User(String userId, String userName, List<Address> addressList, String[] emails) {
            this.userId = userId;
            this.userName = userName;
            this.addressList = addressList;
            this.emails = emails;
        }
    }

    class Address{
        private String province;
        private String city;
        private String detail;

        public Address(String province, String city, String detail) {
            this.province = province;
            this.city = city;
            this.detail = detail;
        }
    }

}