Gson 在尝试解析 List 时抛出 IllegalStateException

Question

我正在学习数据持久化，这是我第一次尝试 JSON。我已经阅读了一些指南，从我所知道的一点点可以看出，在两次存储对象的尝试中，代码都是正确的。我得到了使用 Gson 编写的文件，但是 Gson 在尝试使用 fromJson() 方法解析对象时抛出异常。我的问题如下：

如果我使用相同的类型来转换 to/from JSON 我缺少什么可以告诉 Gson 如何正确解析我的对象？

我尝试了三种不同的方法，下面介绍了其中两种。首先，我尝试将包装器 class 存储在指南建议我应该能够执行的对象列表中：

public class JSONConverter {
    private static Path path = Paths.get("src\json\JSONList.json");
    private static Type stockType = new TypeToken<StocksList>(){}.getType();

    public static void convertToJSON(StocksList stocks, Path path) {
        Gson json = new Gson();     
        String storedStocks = json.toJson(stocks, stockType);// I also tried "StocksList.class" here            
        checkForFile(path);
        try (BufferedWriter writer = Files.newBufferedWriter(path)) {
            writer.write(storedStocks);
        } catch (IOException e) {
            e.printStackTrace();
            //handle later
        }
    }

    static void checkForFile(Path path) {
        if (Files.notExists(path)) {
            try {
                Files.createFile(path);
            } catch (IOException e) {
                e.printStackTrace();
                //handle later
            }
        }
    }

    public static StocksList convertFromJSON(Path path) {
        StocksList stocksList = new StocksList();
        Gson json = new Gson();
        String fromJson;
        try {
            fromJson = Files.readAllBytes(path).toString();
            stocksList = json.fromJson(fromJson, stockType);
            return stocksList;
        } catch (IOException e) {
            return stocksList;
        }
    }
}

我的第二种方法是从包装器中取出列表 class 并尝试将其转换为 JSON:

public class JSONConverter {
    private static Path path = Paths.get("src\json\JSONList.json");
    private static Type listType = new TypeToken<List<Stock>>(){}.getType();

    public static void convertToJSON(StocksList stocks, Path path) {
        Gson json = new Gson();     
        List<Stock> temp = stocks.getStocks();
        String storedStocks = json.toJson(temp, listType);
        checkForFile(path);
        try (BufferedWriter writer = Files.newBufferedWriter(path)) {
            writer.write(storedStocks);
        } catch (IOException e) {
            e.printStackTrace();
            //handle later
        }
    }

    static void checkForFile(Path path) {
        if (Files.notExists(path)) {
            try {
                Files.createFile(path);
            } catch (IOException e) {
                e.printStackTrace();
                //handle later
            }
        }
    }


    public static StocksList convertFromJSON(Path path) {
        StocksList stocksList = new StocksList();
        List<Stock> stocks = new ArrayList<>();
        Gson json = new Gson();
        String fromJson;
        try {
            fromJson = Files.readAllBytes(path).toString();
            stocks = json.fromJson(fromJson, listType);
            //wraps the list in the stockslist class
            stocksList.setStocks(stocks);
            return stocksList;
        } catch (IOException e) {
            e.printStackTrace();
            return stocksList;
        }
    }
}

这是第一种方法使用第二种方法编写的JSON示例。第一个看起来像它除了添加“{ "stocks" :”（你在下面看到的）“}”:

[
{
    "ticker": "INTC",
    "currentPrice": "45.94",
    "marginOfSafety": 0.25,
    "lastDate": "2019-12-28",
    "cashYield": "7.4",
    "MCap": "196485365760",
    "enterpriseValue": "281213850000",
    "sharesOut": "4417000000",
    "oddPercentGrowth": false,
    "newCompany": false,
    "safeValue": "51.35",
    "fairValue": "68.47",
    "evEbitda": "8.56",
    "fcf": [
        "16932000000",
        "14611750000"
    ],
    "rOnAssets": "21",
    "rOnCapital": "20",
    "croic": "16.47",
    "equityToDebt": "3.0",
    "cashOnHand": "4194000000",
    "cashToDebt": "0.17",
    "changeInDebt": "210000000",
    "capEfficiency": [
        "18",
        "7",
        "-26",
        "-21",
        "1"
    ],
    "fcfChange": [
        "18.81",
        "11.71"
    ],
    "profitMargin": [
        "46",
        "38"
    ]
},
{
    "ticker": "HCC",
    "currentPrice": "12.99",
    "marginOfSafety": 0.5,
    "lastDate": "2018-12-31",
    "cashYield": "46.1",
    "MCap": "664587904",
    "enterpriseValue": "1572623480",
    "sharesOut": "52812000",
    "oddPercentGrowth": true,
    "newCompany": true,
    "safeValue": "236.94",
    "fairValue": "473.87",
    "evEbitda": "2.59",
    "fcf": [
        "457776000",
        "306126750"
    ],
    "rOnAssets": "49",
    "rOnCapital": "59",
    "croic": "38.77",
    "equityToDebt": "1.0",
    "cashOnHand": "205577000",
    "cashToDebt": "0.44",
    "changeInDebt": "125283000",
    "capEfficiency": [
        "292",
        "798",
        "-365",
        "-397",
        "-1"
    ],
    "fcfChange": [
        "33.9",
        "33.9"
    ],
    "profitMargin": [
        "40",
        "8"
    ]
}
]

两者都抛出：

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 12

（使用第一种方法时，此行变为 "Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 2"）。在

com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:176) ...

我打算尝试将每个对象单独添加到一个 JSON 数组中，但是当我开始在那里遇到麻烦时，我想我应该问问。该指南提到反射很重要，我猜我的问题在于堆栈跟踪的第二行，但同样，这是我第一次尝试使用 JSON。如果我忘了包含任何内容，请告诉我，我会在评论中 post。

感谢您的帮助。

附录：对象仅在写入文件和从文件中提取时抛出这些异常。在转换为 JSON 字符串然后再转换回来时，它们不会抛出。无论我使用 Files.write() 还是 Files.newBufferedWriter().

都会发生

Answer 1

感谢所有看过我问题的人。我联系了 Gson 的 github 页面，他们对我的 class 进行了以下更正：

All the code you've provided can be greatly fixed, improved and refactored.

No need to create multiple Gson instances: they are relatively expensive to instantiate, but are designed to be thread-safe and immutable therefore can be reused. No need to serialize to and deserialize from java.lang.String -- this is just expensive as long as it has to create multiple strings in the heap merely wasting the heap and time decreasing the performance. Why it does not work in your case is that Files.readAllBytes(...) returns byte[] you're trying to convert to a string. In Java, no arrays have an intuitive toString implementation (you can check it by simply printing any byte array to System.out). In order to convert it to a string (that might be a memory-consuming instance), new String(byte[]) (or even new String(byte[], Charset)) is an appropriate way. I don't really remember how Files works, but there's probably no need to check the file to exist: they can be overwritten without any additional checks. No type tokens are necessary in this case: StockList.class is a Type too. Essentially, all is you need is just as follows:

private static final Gson gson = new GsonBuilder()
    .disableHtmlEscaping()
    .disableInnerClassSerialization()
    .create();

public static void main(final String... args)
        throws IOException {
    final StocksList before = new StocksList(ImmutableList.of(new Stock("INTC"), new 
 Stock("HCC")));
    final Path path = Paths.get("doc.json");
    write(path, before);
    final StocksList after = read(path);
    System.out.println(after.equals(before));
}

private static void write(final Path path, final StocksList stocks)
        throws IOException {
    try ( final Writer writer = new OutputStreamWriter(new 
FileOutputStream(path.toFile())) ) {
        gson.toJson(stocks, writer);
     }
}

 private static StocksList read(final Path path)
        throws IOException {
    try ( final Reader reader = new InputStreamReader(new 
 FileInputStream(path.toFile())) ) {
         return gson.fromJson(reader, StocksList.class);
    }
}

感谢 lyubomyr-shaydariv（Gson 贡献者）的回答。

Gson 在尝试解析 List 时抛出 IllegalStateException

Gson throws IllegalStateException when attempting to parse List

java

json

list

gson