为什么在使用Base64加密给java.lang.ClassCastException时使用Long.valueOf（stateMap.get（“ time”））

Question

For understanding purpose I am combining three different java file code in on Test.java file below. Line long tokenTime = Long.valueOf(decodedMap.get("time")); giving exception.

public class Test {

public static void main(String[] args) {

    Map<String, Long> stateMap = new HashMap<>();
    stateMap.put("userId",123L);
    stateMap.put("resId", 456L);
    stateMap.put("time", System.currentTimeMillis());

    String encodedString = jsonBase64Encode(stateMap);

    Map<String, Long> decodedMap = jsonBase64Decode(encodedString);
    long tokenTime = Long.valueOf(decodedMap.get("time"));
    System.out.println(tokenTime);

}


@SuppressWarnings("unchecked")
public static String jsonBase64Encode(Map<String, Long> stateMap){

    if(stateMap == null)
        return "";
    JSONObject jsonObject = new JSONObject();
    for( String key : stateMap.keySet() ){
        jsonObject.put(key, stateMap.get(key).toString());
    }
    byte[] jsonBytes = jsonObject.toString().getBytes();
    try {
        return Base64.getEncoder().encodeToString(jsonBytes);
    } catch (Exception e) {
        return "";
    }
}


public static Map<String, Long> jsonBase64Decode(String state)
{
    if(StringUtils.isBlank(state))
        return null;
    String decodedStr = null;
    try {
        byte[] decodedBytes = Base64.getDecoder().decode(state);
        decodedStr = new String(decodedBytes);
    } catch (Exception e) {
        return null;
    }
    if(StringUtils.isEmpty(decodedStr))
        return null;
    Map<String, Long> stateMap = new Gson().fromJson(decodedStr, 
            new TypeToken<HashMap<String, Object>>() {}.getType());
    return stateMap;
}

}

I am getting below error.

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
at com.test.document.utils.Test.main(Test.java:26)

Answer 1

That means your runtime map is returning a string when your code was compiled with it expecting a Long:

decodedMap.get("time") //At compile time, this is a Long

But when the code runs at runtime, the actual value is a String.

You can replicate it easily by pushing a string into the map:

stateMap.put("resId", 456L);
((Map) stateMap).put("time", String.valueOf(System.currentTimeMillis()));

long tokenTime = Long.valueOf(stateMap.get("time"));

The last line above raises:

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long

Your JSON needs to be fixed, or you need to change your code to compile against Long.valueOf(String)

Answer 2

Change

Map<String, Long> stateMap = new Gson().fromJson(decodedStr, new TypeToken<HashMap<String, Object>>() {}.getType());

to

Map<String, Long> stateMap = new Gson().fromJson(decodedStr, new TypeToken<HashMap<String, Long>>() {}.getType());