使用gson解析嵌套json

Question

我正在尝试使用Gson将嵌套的json数据解析为Java对象，并使用自定义的toString((将其打印出来。

Json内容

{"data": [
{
"symbol": "BTC",
"circulating_supply": 18975050,
"last_updated": "2022-03-05T13:54:00.000Z",
"total_supply": 18975050,
"cmc_rank": 1,
"self_reported_circulating_supply": null,
"platform": null,
"tags": [
"mineable",
"pow",
"sha-256",
"store-of-value",
"state-channel",
"coinbase-ventures-portfolio",
"three-arrows-capital-portfolio",
"polychain-capital-portfolio",
"binance-labs-portfolio",
"blockchain-capital-portfolio",
"boostvc-portfolio",
"cms-holdings-portfolio",
"dcg-portfolio",
"dragonfly-capital-portfolio",
"electric-capital-portfolio",
"fabric-ventures-portfolio",
"framework-ventures-portfolio",
"galaxy-digital-portfolio",
"huobi-capital-portfolio",
"alameda-research-portfolio",
"a16z-portfolio",
"1confirmation-portfolio",
"winklevoss-capital-portfolio",
"usv-portfolio",
"placeholder-ventures-portfolio",
"pantera-capital-portfolio",
"multicoin-capital-portfolio",
"paradigm-portfolio"
],
"date_added": "2013-04-28T00:00:00.000Z",
"quote": {"USD": {
"fully_diluted_market_cap": 820131199762.62,
"last_updated": "2022-03-05T13:54:00.000Z",
"market_cap_dominance": 42.252,
"percent_change_30d": 6.81621153,
"percent_change_1h": -0.42008562,
"percent_change_24h": -5.71623656,
"market_cap": 741049072478.8419,
"volume_change_24h": -11.0241,
"price": 39053.8666553628,
"percent_change_60d": -16.6843952,
"volume_24h": 26123993408.855915,
"percent_change_90d": -21.03603335,
"percent_change_7d": -0.10097865
}},
"num_market_pairs": 9214,
"name": "Bitcoin",
"max_supply": 21000000,
"id": 1,
"self_reported_market_cap": null,
"slug": "bitcoin"
},
{
"symbol": "ETH",
"circulating_supply": 119834013.5615,
"last_updated": "2022-03-05T13:54:00.000Z",
"total_supply": 119834013.5615,
"cmc_rank": 2,
"self_reported_circulating_supply": null,
"platform": null,
"tags": [
"mineable",
"pow",
"smart-contracts",
"ethereum-ecosystem",
"coinbase-ventures-portfolio",
"three-arrows-capital-portfolio",
"polychain-capital-portfolio",
"binance-labs-portfolio",
"blockchain-capital-portfolio",
"boostvc-portfolio",
"cms-holdings-portfolio",
"dcg-portfolio",
"dragonfly-capital-portfolio",
"electric-capital-portfolio",
"fabric-ventures-portfolio",
"framework-ventures-portfolio",
"hashkey-capital-portfolio",
"kenetic-capital-portfolio",
"huobi-capital-portfolio",
"alameda-research-portfolio",
"a16z-portfolio",
"1confirmation-portfolio",
"winklevoss-capital-portfolio",
"usv-portfolio",
"placeholder-ventures-portfolio",
"pantera-capital-portfolio",
"multicoin-capital-portfolio",
"paradigm-portfolio",
"injective-ecosystem",
"bnb-chain"
],
"date_added": "2015-08-07T00:00:00.000Z",
"quote": {"USD": {
"fully_diluted_market_cap": 316405371023.81,
"last_updated": "2022-03-05T13:54:00.000Z",
"market_cap_dominance": 18.0363,
"percent_change_30d": 1.39750424,
"percent_change_1h": -0.82066438,
"percent_change_24h": -3.20839994,
"market_cap": 316405371023.81366,
"volume_change_24h": -14.3764,
"price": 2640.363629825611,
"percent_change_60d": -31.31982292,
"volume_24h": 12406258074.91051,
"percent_change_90d": -37.45316954,
"percent_change_7d": -4.71064229
}},
"num_market_pairs": 5543,
"name": "Ethereum",
"max_supply": null,
"id": 1027,
"self_reported_market_cap": null,
"slug": "ethereum"
},
and so on...
]}

POJO类：示例

package controller;
import java.util.List;
import javax.annotation.processing.Generated;
import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;
@Generated("jsonschema2pojo")
public class Example {
@SerializedName("data")
@Expose
private List<JsonFormatter> data = null;
public List<JsonFormatter> getData() {
return data;
}
public void setData(List<JsonFormatter> data) {
this.data = data;
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append(Example.class.getName()).append('@').append(Integer.toHexString(System.identityHashCode(this))).append('[');
sb.append("data");
sb.append('=');
sb.append(((this.data == null)?"<null>":this.data));
sb.append(',');
if (sb.charAt((sb.length()- 1)) == ',') {
sb.setCharAt((sb.length()- 1), ']');
} else {
sb.append(']');
}
return sb.toString();
}
}

POJO类：JsonFormatter

package controller;

import java.util.List;
import javax.annotation.processing.Generated;
import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;
@Generated("jsonschema2pojo")
public class JsonFormatter {
@SerializedName("symbol")
@Expose
private String symbol;
@SerializedName("circulating_supply")
@Expose
private Float circulatingSupply;
@SerializedName("last_updated")
@Expose
private String lastUpdated;
@SerializedName("total_supply")
@Expose
private Float totalSupply;
@SerializedName("cmc_rank")
@Expose
private Float cmcRank;
@SerializedName("self_reported_circulating_supply")
@Expose
private Object selfReportedCirculatingSupply;
@SerializedName("platform")
@Expose
private Platform platform;
@SerializedName("tags")
@Expose
private List<String> tags = null;
@SerializedName("date_added")
@Expose
private String dateAdded;
@SerializedName("quote")
@Expose
private Quote quote;
@SerializedName("num_market_pairs")
@Expose
private Float numMarketPairs;
@SerializedName("name")
@Expose
private String name;
@SerializedName("max_supply")
@Expose
private Float maxSupply;
@SerializedName("id")
@Expose
private Integer id;
@SerializedName("self_reported_market_cap")
@Expose
private Object selfReportedMarketCap;
@SerializedName("slug")
@Expose
private String slug;
public String getSymbol() {
return symbol;
}
public void setSymbol(String symbol) {
this.symbol = symbol;
}
public Float getCirculatingSupply() {
return circulatingSupply;
}
public void setCirculatingSupply(Float circulatingSupply) {
this.circulatingSupply = circulatingSupply;
}
public String getLastUpdated() {
return lastUpdated;
}
public void setLastUpdated(String lastUpdated) {
this.lastUpdated = lastUpdated;
}
public Float getTotalSupply() {
return totalSupply;
}
public void setTotalSupply(Float totalSupply) {
this.totalSupply = totalSupply;
}
public Float getCmcRank() {
return cmcRank;
}
public void setCmcRank(Float cmcRank) {
this.cmcRank = cmcRank;
}
public Object getSelfReportedCirculatingSupply() {
return selfReportedCirculatingSupply;
}
public void setSelfReportedCirculatingSupply(Object selfReportedCirculatingSupply) {
this.selfReportedCirculatingSupply = selfReportedCirculatingSupply;
}
public Platform getPlatform() {
return platform;
}
public void setPlatform(Platform platform) {
this.platform = platform;
}
public List<String> getTags() {
return tags;
}
public void setTags(List<String> tags) {
this.tags = tags;
}
public String getDateAdded() {
return dateAdded;
}
public void setDateAdded(String dateAdded) {
this.dateAdded = dateAdded;
}
public Quote getQuote() {
return quote;
}
public void setQuote(Quote quote) {
this.quote = quote;
}
public Float getNumMarketPairs() {
return numMarketPairs;
}
public void setNumMarketPairs(Float numMarketPairs) {
this.numMarketPairs = numMarketPairs;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Float getMaxSupply() {
return maxSupply;
}
public void setMaxSupply(Float maxSupply) {
this.maxSupply = maxSupply;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Object getSelfReportedMarketCap() {
return selfReportedMarketCap;
}
public void setSelfReportedMarketCap(Object selfReportedMarketCap) {
this.selfReportedMarketCap = selfReportedMarketCap;
}
public String getSlug() {
return slug;
}
public void setSlug(String slug) {
this.slug = slug;
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append(JsonFormatter.class.getName()).append('@').append(Integer.toHexString(System.identityHashCode(this))).append('[');
sb.append("symbol");
sb.append('=');
sb.append(((this.symbol == null)?"<null>":this.symbol));
sb.append(',');
sb.append("circulatingSupply");
sb.append('=');
sb.append(((this.circulatingSupply == null)?"<null>":this.circulatingSupply));
sb.append(',');
sb.append("lastUpdated");
sb.append('=');
sb.append(((this.lastUpdated == null)?"<null>":this.lastUpdated));
sb.append(',');
sb.append("totalSupply");
sb.append('=');
sb.append(((this.totalSupply == null)?"<null>":this.totalSupply));
sb.append(',');
sb.append("cmcRank");
sb.append('=');
sb.append(((this.cmcRank == null)?"<null>":this.cmcRank));
sb.append(',');
sb.append("selfReportedCirculatingSupply");
sb.append('=');
sb.append(((this.selfReportedCirculatingSupply == null)?"<null>":this.selfReportedCirculatingSupply));
sb.append(',');
sb.append("platform");
sb.append('=');
sb.append(((this.platform == null)?"<null>":this.platform));
sb.append(',');
sb.append("tags");
sb.append('=');
sb.append(((this.tags == null)?"<null>":this.tags));
sb.append(',');
sb.append("dateAdded");
sb.append('=');
sb.append(((this.dateAdded == null)?"<null>":this.dateAdded));
sb.append(',');
sb.append("quote");
sb.append('=');
sb.append(((this.quote == null)?"<null>":this.quote));
sb.append(',');
sb.append("numMarketPairs");
sb.append('=');
sb.append(((this.numMarketPairs == null)?"<null>":this.numMarketPairs));
sb.append(',');
sb.append("name");
sb.append('=');
sb.append(((this.name == null)?"<null>":this.name));
sb.append(',');
sb.append("maxSupply");
sb.append('=');
sb.append(((this.maxSupply == null)?"<null>":this.maxSupply));
sb.append(',');
sb.append("id");
sb.append('=');
sb.append(((this.id == null)?"<null>":this.id));
sb.append(',');
sb.append("selfReportedMarketCap");
sb.append('=');
sb.append(((this.selfReportedMarketCap == null)?"<null>":this.selfReportedMarketCap));
sb.append(',');
sb.append("slug");
sb.append('=');
sb.append(((this.slug == null)?"<null>":this.slug));
sb.append(',');
if (sb.charAt((sb.length()- 1)) == ',') {
sb.setCharAt((sb.length()- 1), ']');
} else {
sb.append(']');
}
return sb.toString();
}
}

我的POJO还有其他类，我试图将其解析为Gson的方法是：

Gson gson = new Gson();
Type type = new TypeToken<Map<String, ArrayList<JsonFormatter>>>() {}.getType();
Map<String, ArrayList<Example>> nodeProperties = gson.fromJson(json.toString(), type);

但当我尝试打印出它的子值时，比如

for (Map.Entry<String, ArrayList<Example>> set : nodeProperties.entrySet()) {
System.out.println(set.getValue().get(0).getData().get(0).toString());
}

我得到错误：

class controller.JsonFormatter cannot be cast to class controller.Example (controller.JsonFormatter and controller.Example are in unnamed module of loader 'app')

我想要实现的是将上面的json内容打印如下：

Symbol: BTC
Circulating_supply: 18975050
...
Tags:
mineable

有人能帮我解决这个问题吗？提前感谢！

Answer 1

问题出现在以下几行：

Type type = new TypeToken<Map<String, ArrayList<JsonFormatter>>>() {}.getType();
Map<String, ArrayList<Example>> nodeProperties = gson.fromJson(json.toString(), type);

TypeToken用于Map<String, ArrayList<JsonFormatter>>，但您将解析的JSON存储在类型为Map<String, ArrayList<Example>>的变量中。不幸的是，Gson.fromJson(..., Type)没有提供任何类型安全保证，因此您必须确保提供的Type与您存储结果的变量的类型匹配。此外，由于Java在运行时的类型擦除，您不会注意到JsonFormatter与Example不匹配，直到您迭代元素，导致ClassCastException。

根据您提供的JSON数据，在我看来您可以只使用gson.fromJson(json.toString(), Example.class)，但请验证这是否真的适用于您的用例。