java - 如何使用 Gson 在 Java 中表示具有混合数据类型的 Json 对象

Question

我在将一些 Json 序列化为 Java 对象时遇到了一些困难。正如您所看到的，Json 位包含顶级元素 args 以及包含 String 作为键和混合值包的对象列表。我在 Java 源文件中尝试了一些创造性的东西:

Json

{
    "args": {
        "all": {
            "expectCliArgs": true
        },
        "Prog3a": {
            "expectedNumberOfCliArgs": 2,
            "expectedErrorMessage": "Usage: java Prog3a infilename searchword"
        },
        "Prog3b": {
            "expectedNumberOfCliArgs": 2,
            "expectedErrorMessage": "Usage: java Prog3b infileName outfileName"
        },
        "Prog3c": {
            "expectedNumberOfCliArgs": 1,
            "expectedErrorMessage": "Usage: java Prog3c infileName"
        }
    }
}

Java

protected class Args {
    public Map<String, Set<Map<String, Object>>> map = new HashMap<>(); // no dice
}

protected class Args {
    public Map<String, Set<Map<String, ?>>> map = new HashMap<>(); // no dice
}

我是否需要 custom serializer ？

更新我尝试在类里面执行以下操作(按照 SO 答案):

private Map<String, Set<ExpectedArgs>> args = new HashMap<>();

protected class ExpectedArgs {
    public Boolean expectCliArgs;
    public Integer expectedNumberOfCliArgs;
    public String expectedArgsErrorMessage;
}

但现在看来 Gson 正在提示我的 Json 文件的语法:

Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 847
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:176)
    at com.google.gson.Gson.fromJson(Gson.java:803)
    at com.google.gson.Gson.fromJson(Gson.java:768)
    at com.google.gson.Gson.fromJson(Gson.java:717)
    at com.google.gson.Gson.fromJson(Gson.java:689)
    at javaGrader.config.ParseConfig.convertJsonToObject(ParseConfig.java:72)
    at javaGrader.config.ParseConfig.parseConfig(ParseConfig.java:34)
    at javaGrader.JavaGrader.main(JavaGrader.java:60)
Disconnected from the target VM, address: '127.0.0.1:62344', transport: 'socket'
Caused by: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 847
    at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:338)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:79)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:60)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:40)
    at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:187)
    at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:93)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:172)
    ... 7 more

我在 Json 文件中设置了一些断点，看起来“args”对象是导致胃灼热的原因。作为引用，这里是 full Json file , java Config class ，和the class where Gson runs 。看来 Gson 将我的对象与数组混淆了。

第 847 栏是:

searchword"},"Prog3b":{
             ^

有什么想法吗？

更新2我的变量签名不正确:

private Map<String, Set<ExpectedArgs>> args = new HashMap<>();  // wrong
private Map<String, ExpectedArgs> args = new HashMap<>(); // right

Answer 1

我将为包含两组字段的内部对象定义一个类，如下所示:

public static class ExpectedArgs {
    public Boolean expectCliArgs;
    public Integer expectedNumberOfCliArgs;
    public String expectedErrorMessage;
}

(注意，我在这里故意使用了 Boolean 和 Integer 而不是 boolean 和 int，这样你可以通过测试 null 来判断该字段是否已设置。)

然后你可以像这样定义外部类:

public static class Data {
    public Map<String, ExpectedArgs> args;
}

最后，您可以将 JSON 反序列化为 Data 实例:

String json = "...";

Gson gson = new Gson();

Data data = gson.fromJson(json, Data.class);

data.args.forEach((key, value) -> {
    System.out.println(key);
    System.out.println(value.expectCliArgs);
    System.out.println(value.expectedNumberOfCliArgs);
    System.out.println(value.expectedErrorMessage);
    System.out.println();
});

此代码为您的示例 JSON 输出以下内容:

all
true
null
null

Prog3a
null
2
Usage: java Prog3a infilename searchword

Prog3b
null
2
Usage: java Prog3b infileName outfileName

Prog3c
null
1
Usage: java Prog3c infileName