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上篇文章gson用户指南(上)翻译到了10、序列化和反序列化有任意类型对象的collection
我们继续
java.net.URL to match it with strings like "https://github.com/google/gson/" java.net.URI to match it with strings like "/google/gson/"你可以从源码中找到一些常用的类。例如JodaTime 点击打开链接
GsonBuilder gson = new GsonBuilder(); gson.registerTypeAdapter(MyType2.class, new MyTypeAdapter()); gson.registerTypeAdapter(MyType.class, new MySerializer()); gson.registerTypeAdapter(MyType.class, new MyDeserializer()); gson.registerTypeAdapter(MyType.class, new MyInstanceCreator());registerTypeAdapter调用检查类型适配器实现一个以上的接口,注册它。
private class DateTimeSerializer implements JsonSerializer<DateTime> { public JsonElement serialize(DateTime src, Type typeOfSrc, JsonSerializationContext context) { return new JsonPrimitive(src.toString()); } }当它运行到一个DateTime对象序列化时。Gson会调用serialize()。
private class DateTimeDeserializer implements JsonDeserializer<DateTime> { public DateTime deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException { return new DateTime(json.getAsJsonPrimitive().getAsString()); } }当需要一个JSON字符串反序列化
private class MoneyInstanceCreator implements InstanceCreator<Money> { public Money createInstance(Type type) { return new Money("1000000", CurrencyCode.USD); } }Type 可以是相应的泛型
class MyList<T> extends ArrayList<T> { } class MyListInstanceCreator implements InstanceCreator<MyList<?>> { @SuppressWarnings("unchecked") public MyList<?> createInstance(Type type) { // No need to use a parameterized list since the actual instance will have the raw type anyway. return new MyList(); } }然而,有时你需要根据实际参数化类型创建实例。这种情况下,可以使用传递到createInstance方法的类型参数,例如
public class Id<T> { private final Class<T> classOfId; private final long value; public Id(Class<T> classOfId, long value) { this.classOfId = classOfId; this.value = value; } } class IdInstanceCreator implements InstanceCreator<Id<?>> { public Id<?> createInstance(Type type) { Type[] typeParameters = ((ParameterizedType)type).getActualTypeArguments(); Type idType = typeParameters[0]; // Id has only one parameterized type T return Id.get((Class)idType, 0L); } }在上面的示例中,如果没有传递参数化类型的真实类型,不能创建Id类的实例。我们通过方法参数传递类型type来解决这个问题。在本例中Java对象是类型参数化的类型表示的Id < Foo >应该绑定到实例Id < Foo >。由于Id类只有一个参数化的类型参数T,我们使用getActualTypeArgument()返回的第0个元素类型的数组将持有Foo.class
Gson gson = new GsonBuilder().setPrettyPrinting().create(); String jsonOutput = gson.toJson(someObject);
Gson gson = new GsonBuilder().serializeNulls().create();注意,当使用Gson序列化null对象时,会在JsonElement 结构中增加JsonNull ,这个对象不能用于自定义序列化反序列化
public class Foo { private final String s; private final int i; public Foo() { this(null, 5); } public Foo(String s, int i) { this.s = s; this.i = i; } } Gson gson = new GsonBuilder().serializeNulls().create(); Foo foo = new Foo(); String json = gson.toJson(foo); System.out.println(json); json = gson.toJson(null); System.out.println(json);输出
{"s":null,"i":5} null
public class VersionedClass { @Since(1.1) private final String newerField; @Since(1.0) private final String newField; private final String field; public VersionedClass() { this.newerField = "newer"; this.newField = "new"; this.field = "old"; } } VersionedClass versionedObject = new VersionedClass(); Gson gson = new GsonBuilder().setVersion(1.0).create(); String jsonOutput = gson.toJson(someObject); System.out.println(jsonOutput); System.out.println(); gson = new Gson(); jsonOutput = gson.toJson(someObject); System.out.println(jsonOutput);
{"newField":"new","field":"old"} {"newerField":"newer","newField":"new","field":"old"}
import java.lang.reflect.Modifier; Gson gson = new GsonBuilder() .excludeFieldsWithModifiers(Modifier.STATIC) .create();可以添加多个常量
Gson gson = new GsonBuilder() .excludeFieldsWithModifiers(Modifier.STATIC, Modifier.TRANSIENT, Modifier.VOLATILE) .create();
这个特性提供了一种方法,可以标记某些字段的对象排除序列化和反序列化为JSON。使用这个注解,必须调用new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create()。这样@Expose注解的类和字段才能被序列化和反序列化。
如果排除字段和类类型上述机制不适合你,那么你可以编写自己的排斥战略,并把它应用到GSON。详细信息请参照文档,文档地址ExclusionStragegy
示例
@Retention(RetentionPolicy.RUNTIME) @Target({ElementType.FIELD}) public @interface Foo { // Field tag only annotation } public class SampleObjectForTest { @Foo private final int annotatedField; private final String stringField; private final long longField; private final Class<?> clazzField; public SampleObjectForTest() { annotatedField = 5; stringField = "someDefaultValue"; longField = 1234; } } public class MyExclusionStrategy implements ExclusionStrategy { private final Class<?> typeToSkip; private MyExclusionStrategy(Class<?> typeToSkip) { this.typeToSkip = typeToSkip; } public boolean shouldSkipClass(Class<?> clazz) { return (clazz == typeToSkip); } public boolean shouldSkipField(FieldAttributes f) { return f.getAnnotation(Foo.class) != null; } } public static void main(String[] args) { Gson gson = new GsonBuilder() .setExclusionStrategies(new MyExclusionStrategy(String.class)) .serializeNulls() .create(); SampleObjectForTest src = new SampleObjectForTest(); String json = gson.toJson(src); System.out.println(json); }
{"longField":1234}
Gson支持一些预先定义的字段命名策略转换标准的Java字段名,例如小写字母开头的驼峰式命名,详细信息可以参考文档FieldNamingPolicy
它也有一个基于注解的策略,允许客户端自定义名称。请注意,基于策略的注解有字段名的验证,如果一个无效的字段名称作为注解将增加运行异常。
下面是一个自定义命名策略的例子
private class SomeObject { @SerializedName("custom_naming") private final String someField; private final String someOtherField; public SomeObject(String a, String b) { this.someField = a; this.someOtherField = b; } } SomeObject someObject = new SomeObject("first", "second"); Gson gson = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE).create(); String jsonRepresentation = gson.toJson(someObject); System.out.println(jsonRepresentation);
{"custom_naming":"first","SomeOtherField":"second"}
http://groups.google.com/group/google-gson/browse_thread/thread/cb441a2d717f6892
http://google.github.io/gson/apidocs/com/google/gson/annotations/SerializedName.html
有时候需要共享序列化反序列化,可以用下面三个方法
(1)通过静态字段存储共享状态
(2)在父类中定义序列化反序列化内部类,并使用父类型的实例字段来存储共享状态。
(3)使用ThreadLocal
1,2是线程不安全的,3是线程安全的
除了Gson的对象模型和数据绑定,可使用GSON读取和写入流。也可以流和对象模型的访问都是用,以获得最佳的方法。
见GSON设计文档,对设计GSON时我们面临的问题进行了探讨。也包括GSON与可用于JSON的转换其他Java库的比较。
参考地址https://sites.google.com/site/gson/gson-design-document
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原文地址:http://blog.csdn.net/robertcpp/article/details/51599124