gson-GsonBuilder

本是古典何须时尚 2022-02-19 04:27 198阅读 0赞

[资源贴][Link 1]

### 文章目录 ###

*   *   *  1. 默认排除策略
         *  2. 自定义排除策略
         *  3. 序列化空字段
         *  4. 类型适配器
         *  5. 字段命名策略
         *  6. 其它相关方法

描述：当您需要设置默认配置选项以外的配置选项时，请使用此生成器构造Gson实例。对于具有默认配置的Gson，使用new Gson() 更简单。GsonBuilder的最佳用法是创建它，然后调用它的各种配置方法，最后调用create。

### 1. 默认排除策略 ###

*  excludeFieldsWithModifiers(int… modifiers) 序列化或反序列化排除特定修饰语的字段
 *  excludeFieldsWithoutExposeAnnotation()序列化或反序列化排除没有@Expoe的字段前面注解中已经讲述，这边不讲

例子：排除指定修饰器修饰的字段  
修饰器可以指定下述这些字段

*  PUBLIC,
 *  PROTECTED,
 *  PRIVATE,
 *  ABSTRACT,
 *  DEFAULT,
 *  STATIC,
 *  FINAL,
 *  TRANSIENT,
 *  VOLATILE,
 *  SYNCHRONIZED,
 *  NATIVE,
 *  STRICTFP;

默认排除static或transient描述的字段

public class SomeObject { 
            private static String field1;
            private transient String field2;
            private String field3;
            private String field4;
        }
        
        public static void main(String[] args) throws Exception { 
            SomeObject someObject = new SomeObject();
            someObject.setField1("field1");
            someObject.setField2("field2");
            someObject.setField3("field3");
            someObject.setField4("field4");
            Gson gson = new GsonBuilder().create();
            System.out.println(gson.toJson(someObject));
        }
        // ==>{"field3":"field3","field4":"field4"}

也可以自己指定排除字段

public static void main(String[] args) throws Exception { 
            SomeObject someObject = new SomeObject();
            someObject.setField1("field1");
            someObject.setField2("field2");
            someObject.setField3("field3");
            someObject.setField4("field4");
            Gson gson = new GsonBuilder().excludeFieldsWithModifiers(Modifier.STATIC).create();
            System.out.println(gson.toJson(someObject));
        }
        // ==>{"field2":"field2","field3":"field3","field4":"field4"}

--------------------

### 2. 自定义排除策略 ###

方法：

*  addSerializationExclusionStrategy(ExclusionStrategy strategy) 序列化排除策略
 *  addDeserializationExclusionStrategy(ExclusionStrategy strategy) 反序列化排除策略
 *  setExclusionStrategies(ExclusionStrategy… strategies) 序列化/反序列化排除策略

例子：序列化过程排除不需要字段

a. 前面讲过的@Expose字段已经可以实现，但是@Expose是加在需要序列化的字段上，假设字段需要序列化的字段多，其它少，这种方式比较麻烦；

b. 自己实现注解，排除不必要字段

注解：

@Retention(RetentionPolicy.RUNTIME)
        @Target({ ElementType.FIELD, ElementType.TYPE})
        public @interface GsonTransparent { 
        }

策略实现类

public class GsonTransparentStrategy implements ExclusionStrategy { 
    
       /** * 如果注解@GsonTransparent存在排除该字段 * * @param f * @return */
       @Override
       public boolean shouldSkipField(FieldAttributes f) { 
           return f.getAnnotation(GsonTransparent.class) != null;
       }
    
       /** * 如果注解@GsonTransparent存在排除该类 * * @param clazz * @return */
       @Override
       public boolean shouldSkipClass(Class<?> clazz) { 
           return clazz.getAnnotation(GsonTransparent.class) != null;
       }
    }

使用方式

*  注解在字段上，该字段不被序列化或反序列化
 *  注解在类上，该类不被序列化或反序列化

// 实体类
        public class SomeObject { 
            @GsonTransparent
            private String field1;
            private String field2;
            private String field3;
            private String field4;
        }
        
        // 测试
        public static void main(String[] args) throws Exception { 
            SomeObject someObject = new SomeObject();
            someObject.setField1("field1");
            someObject.setField2("field2");
            someObject.setField3("field3");
            someObject.setField4("field4");
            // 序列化和反序列化注解都生效
            // 其它两个方法分别对应序列化和反序列化
            Gson gson = new GsonBuilder().setExclusionStrategies(new GsonTransparentStrategy()).create();
            System.out.println(gson.toJson(someObject));
        }
        // ==>{"field2":"field2","field3":"field3","field4":"field4"}

--------------------

### 3. 序列化空字段 ###

方法：

*  serializeNulls()

例子：

public static void main(String[] args) throws Exception { 
            SomeObject someObject = new SomeObject();
            someObject.setField1("field1");
            someObject.setField2("field2");
            someObject.setField3("field3");
            Gson gson = new GsonBuilder().serializeNulls().create();
            System.out.println(gson.toJson(someObject));
        }

--------------------

### 4. 类型适配器 ###

方法：

*  registerTypeAdapter(java.lang.reflect.Type type, java.lang.Object typeAdapter) 为type类型字段，指定特殊的类型适配器

例子01：格式化日期类型

@JsonAdapter 一样的方式  
可以支持 JsonDeserializer,JsonSerializer,InstanceCreator,TypeAdapter四种适配器类型

// 适配器类

public class DateAdapter extends TypeAdapter<Date> { 
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
    
        /** * 序列化 * * @param out 输出流 * @param value 值 * @throws IOException */
        @Override
        public void write(JsonWriter out, Date value) throws IOException { 
            if (value != null) { 
                out.value(simpleDateFormat.format(value));
            } else { 
                out.value("");
            }
        }
    
        /** * 反序列化 * * @param in 输入流 * @return * @throws IOException */
        @Override
        public Date read(JsonReader in) throws IOException { 
            String str = in.nextString();
            Date date = null;
            try { 
                date = simpleDateFormat.parse(str);
            } catch (ParseException e) { 
                e.printStackTrace();
            }
            return date;
        }
    }

// 测试类

public class Test0020 { 
        public static void main(String[] args) throws Exception { 
            SomeObject someObject = new SomeObject();
            someObject.setField1("field1");
            someObject.setField2("field2");
            someObject.setField3("field3");
            someObject.setDate(new Date());
            Gson gson = new GsonBuilder().registerTypeAdapter(Date.class, new DateAdapter()).serializeNulls().create();
            System.out.println(gson.toJson(someObject));
        }
    }
    
    // ==>{"field1":"field1","field2":"field2","field3":"field3","field4":null,"date":"2019-04-15 04:01:35"}

注：如果只是要格式化日期，可以通过  
setDateFormat(java.lang.String pattern)设置格式

例子02：String为null转为""输出  
// String类型适配器

public class StringAdapter extends TypeAdapter<String> { 
        @Override
        public void write(JsonWriter out, String value) throws IOException { 
            if (value == null) { 
                out.value("");
            } else { 
                out.value(value);
            }
        }
    
        @Override
        public String read(JsonReader in) throws IOException { 
            return in.nextString();
        }
    }

// 测试类

public class Test0020 { 
        public static void main(String[] args) throws Exception { 
            SomeObject someObject = new SomeObject();
            someObject.setField1("field1");
            someObject.setField2("field2");
            someObject.setField3("field3");
            someObject.setDate(new Date());
            Gson gson = new GsonBuilder().registerTypeAdapter(Date.class, new DateAdapter()).registerTypeAdapter(String.class, new StringAdapter()).serializeNulls().create();
            System.out.println(gson.toJson(someObject));
        }
    }
    // ==> {"field1":"field1","field2":"field2","field3":"field3","field4":"","date":"2019-04-15 04:03:53"}

--------------------

### 5. 字段命名策略 ###

方法：

*  setFieldNamingStrategy(FieldNamingStrategy fieldNamingStrategy)

例子：驼峰转下划线

public class SomeObject { 
            private String helloWord;
        
            public String getHelloWord() { 
                return helloWord;
            }
        
            public void setHelloWord(String helloWord) { 
                this.helloWord = helloWord;
            }
        }
    
        public static void main(String[] args) throws Exception { 
            SomeObject someObject = new SomeObject();
            someObject.setHelloWord("hello world");
            Gson gson = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES).create();
            System.out.println(gson.toJson(someObject));
        }
        // ==>{"hello_word":"hello world"}

### 6. 其它相关方法 ###

上述情况包含了我在项目中使用的大多数情况，还有一些方法比较简单可以参看资源贴。

[Link 1]: https://www.javadoc.io/doc/com.google.code.gson/gson/2.8.5