java - Gson在反序列化对象时忽略null

Question

我想在 Java 中反序列化一个包含空值的 json 字符串。我想将该对象反序列化为 Properties 对象。 json 字符串类似于:

{"prop1":null, "propr2":"fancy value"}

当我反序列化时使用

String json = //
new Gson().fromJson(json, Properties.class);

由于 Properties 对象中的 Hastable，我得到一个空指针异常。如何指示 Gson 忽略空值的反序列化？

Answer 1

问题确实是 Gson 的默认适配器试图将 null 放入 Properties 中，这是被禁止的。

要解决这个问题，您可以编写自己的 TypeAdapter对于 Properties。然后，您必须使用 GsonBuilder 创建 Gson 实例，您 registered那种类型的适配器。

下面显示了这样一个适配器的外观。它稍微更严格一些，因为它在序列化期间防止非字符串键和值(Gson 的默认适配器不这样做)，因为它们会在反序列化期间引起问题。但是，您可以使用 Gson.getDelegateAdapter​ 替换它并将序列化委托(delegate)给 Gson 的适配器。 .

private static final TypeAdapter<Properties> PROPERTIES_ADAPTER = new TypeAdapter<Properties>() {
    @Override
    public Properties read(JsonReader in) throws IOException {
        in.beginObject();

        Properties properties = new Properties();
        while (in.hasNext()) {
            String name = in.nextName();
            JsonToken peeked = in.peek();

            // Ignore null values
            if (peeked == JsonToken.NULL) {
                in.nextNull();
                continue;
            }
            // Allow Json boolean
            else if (peeked == JsonToken.BOOLEAN) {
                properties.setProperty(name, Boolean.toString(in.nextBoolean()));
            }
            // Expect string or number
            else {
                properties.setProperty(name, in.nextString());
            }
        }

        in.endObject();
        return properties;
    }

    private String asString(Object obj) {
        if (obj.getClass() != String.class) {
            throw new IllegalArgumentException("Properties contains non-String object " + obj);
        }
        return (String) obj;
    }

    /*
     * Could also delegate to Gson's implementation for serialization.
     * However, that would not fail if the Properties contains non-String values,
     * which would then cause issues when deserializing the Json again. 
     */
    @Override
    public void write(JsonWriter out, Properties properties) throws IOException {
        out.beginObject();

        for (Map.Entry<Object, Object> entry : properties.entrySet()) {
            // Make sure that key is a String, otherwise properties
            // cannot be deserialized again
            out.name(asString(entry.getKey()));

            Object value = entry.getValue();
            // Be lenient and allow Numbers and Booleans as values
            if (value instanceof Number) {
                out.value((Number) value);
            } else if (value instanceof Boolean) {
                out.value((Boolean) value);
            } else {
                // Require that value is a String
                out.value(asString(value));
            }
        }

        out.endObject();
    }

}.nullSafe(); // Handle null Properties, e.g. `Properties props = null`

public static void main(String[] args) throws IOException {
    Gson gson = new GsonBuilder()
        // Register the custom type adapter
        .registerTypeAdapter(Properties.class, PROPERTIES_ADAPTER)
        .create();

    String json = "{\"prop1\":true, \"prop2\":\"text\", \"prop3\":null}";
    Properties deserialized = gson.fromJson(json, Properties.class); 
    System.out.println("Deserialized: " + deserialized);

    Properties properties = new Properties();
    properties.setProperty("prop", "text");
    // Discouraged to put non-Strings, but type adapter supports these
    properties.put("boolean", true);
    properties.put("number", 1234);
    System.out.println("Serialized: " + gson.toJson(properties));
}