HTTP GET 请求返回 Null？（爪哇）答案

【问题标题】：HTTP GET Request Returning Null? (Java)HTTP GET 请求返回 Null？（爪哇）
【发布时间】：2016-11-08 02:49:53
【问题描述】：

我正在使用以下代码创建一个 HTTP GET 请求：

URL obj = new URL(pl.getConfig().getString("Database URL") + player.getName());
        HttpURLConnection con = (HttpURLConnection) obj.openConnection();
        BufferedReader in = new BufferedReader(
                new InputStreamReader(con.getInputStream()));
        String inputLine;
        StringBuffer response = new StringBuffer();

        while ((inputLine = in.readLine()) != null) {
            response.append(inputLine);
        }
        in.close();
        Logger.getLogger("Minecraft").info(inputLine); //Logs null

        Gson gsonData = new Gson();
        MinecladJson jsonData = gsonData.fromJson(inputLine, MinecladJson.class); //Returns null (because inputLine is null)
        PlayerData jsonPlayerData = jsonData.getPlayer(); //Crash here

不幸的是，变量inputLine 似乎为空，导致我的代码中出现 NullPointerException。更糟糕的是，该地址在我的浏览器中运行良好并返回：

{"player":{"balance":0,"permissions":["ultracosmetics.recievechest","ultracosmetics.openmenu"],"stats":{"playerKills":0,"deaths":0,"minutesPlayed":0},"achievements":{"firstJoin":false},"joinMessage":"&8[&9&l+&r&8] %r&7%p","leaveMessage":"&8[&9&l-&r&8] %r&7%p","rank":"default","firstJoin":"2016-07-01T02:37:14.458Z"},"content":{"chatFormat":"%r&7%p: %m","chatCensor":["testcensor"],"chatPrefixes":{"mage":"&5MAGE ","warrior":"&3WARRIOR ","chief":"&2CHIEF ","sage":"&7SAGE ","default":"","developer":"&9&lDEV ","moderator":"&6MOD "},"currencyName":"Coins","saveRate":30000,"cacheResetRate":1200000,"allowedCharacters":"abcdefghijklmnopqrstuvwxyz0123456789!~*.,/][()=+@#$%\"';:_`^&{}\\|<>?","tabHeader":"&8&lMineclad &r&9Network","tabFooter":"&7Mineclad.co"},"uuid":"559f6d32c2b740c9bc843dd54a55b39c"}

堆栈跟踪：

[09:16:54 WARN]: java.lang.NullPointerException
[09:16:54 WARN]:    at com.techurify.minecladPlugin.event.player.PlayerJoinListener.onPlayerJoin(PlayerJoinListener.java:56)
[09:16:54 WARN]:    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[09:16:54 WARN]:    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[09:16:54 WARN]:    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[09:16:54 WARN]:    at java.lang.reflect.Method.invoke(Method.java:483)
[09:16:54 WARN]:    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:300)
[09:16:54 WARN]:    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
[09:16:54 WARN]:    at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:501)
[09:16:54 WARN]:    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:486)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.PlayerList.onPlayerJoin(PlayerList.java:317)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.PlayerList.a(PlayerList.java:144)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.LoginListener.b(LoginListener.java:115)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.LoginListener.E_(LoginListener.java:53)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.NetworkManager.a(NetworkManager.java:224)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.ServerConnection.c(SourceFile:187)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.MinecraftServer.D(MinecraftServer.java:762)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.DedicatedServer.D(DedicatedServer.java:360)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.MinecraftServer.C(MinecraftServer.java:646)
[09:16:54 WARN]:    at net.minecraft.server.v1_10_R1.MinecraftServer.run(MinecraftServer.java:550)
[09:16:54 WARN]:    at java.lang.Thread.run(Thread.java:745)

我想知道：

为什么会这样
以及如何解决它

【问题讨论】：

请显示完整的堆栈跟踪并在您的代码中指出异常发生的行。
在这段代码中inputLine 不可能为空，除非在读取循环结束时，您不再使用它。
@EJP 是的，这就是我很困惑的原因。
@JimGarrison 更新了！
是的，我登录了inputLine，它是空的。 jsonData 为空，因为 inputLine 为空。在文档中它说：返回：an object of type T from the string. Returns null if json is null.

标签： java json http url

【解决方案1】：

我相信而不是

MinecladJson jsonData = gsonData.fromJson(inputLine, MinecladJson.class);

你想要

MinecladJson jsonData = gsonData.fromJson(response.toString(), MinecladJson.class);
                                          ^^^^^^^^^^^^^^^^^^^

您将单个响应行累积到 response，但随后尝试从 inputLine 中提取对象，此时将始终为 null。

【讨论】：

【解决方案2】：

您正在记录inputLine，但我假设您想记录response。

对于离开 while 循环 inputLine 必须为 null，因此我难怪您在打印 inputLine 时得到该结果：

    StringBuffer response = new StringBuffer();

    while ((inputLine = in.readLine()) != null) { 
        // leave while loop if inputLine == in.readLine() == null

        response.append(inputLine);
    }
    in.close();
    Logger.getLogger("Minecraft").info(inputLine); //Logs null

【讨论】：