非常抱歉，代码有点长，但我想知道是否有人可以帮忙解决一个多线程问题（我对多线程还很陌生）。我正在尝试设计一个RESTful Web服务API的Facade类，可以与多个线程共享。我使用HttpURLConnection进行连接，并使用Google GSON将数据转换为JSON格式。
下面是我目前的代码。在这个例子中，它有一个公共方法来调用API（authenticateCustomer()），私有方法用于实现API调用（例如构建POST数据字符串，发送POST请求等）。
我创建了一个该类的实例，并与1000个线程共享。这些线程调用authenticateCustomer()方法。大多数线程都能正常工作，但有一些线程会出现空指针异常，这是因为我没有实现任何同步。如果我将authenticateCustomer()方法设置为“synchronized”，它就可以正常工作。问题是这会导致并发性差（例如，如果POST请求突然需要很长时间才能完成，这将阻塞所有其他线程）。
现在我的问题是，下面的类不是无状态的吗？因此线程安全？类中仅有极少量的字段，在构造函数中声明为final并进行赋值。所有方法均使用本地变量。Gson对象是无状态的（根据其网站），而且在API方法中也是作为本地变量创建的。
public final class QuizSyncAPIFacade
{
// API Connection Details
private final String m_apiDomain;
private final String m_apiContentType;
private final int m_bufferSize;
// Constructors
public QuizSyncAPIFacade()
{
m_apiDomain = "http://*****************************";
m_apiContentType = ".json";
m_bufferSize = 8192; // 8k
}
private String readInputStream(InputStream stream) throws IOException
{
// Create a buffer for the input stream
byte[] buffer = new byte[m_bufferSize];
int readCount;
StringBuilder builder = new StringBuilder();
while ((readCount = stream.read(buffer)) > -1) {
builder.append(new String(buffer, 0, readCount));
}
return builder.toString();
}
private String buildPostData(HashMap<String,String> postData) throws UnsupportedEncodingException
{
String data = "";
for (Map.Entry<String,String> entry : postData.entrySet())
{
data += (URLEncoder.encode(entry.getKey(), "UTF-8") + "=" + URLEncoder.encode(entry.getValue(), "UTF-8") + "&");
}
// Trim the last character (a trailing ampersand)
int length = data.length();
if (length > 0) {
data = data.substring(0, (length - 1));
}
return data;
}
private String buildJSONError(String message, String name, String at)
{
String error = "{\"errors\":[{\"message\":\"" + message + "\",\"name\":\"" + name + "\",\"at\":\"" + at + "\"}]}";
return error;
}
private String callPost(String url, HashMap<String,String> postData) throws IOException
{
// Set up the URL for the API call
URL apiUrl = new URL(url);
// Build the post data
String data = buildPostData(postData);
// Call the API action
HttpURLConnection conn;
try {
conn = (HttpURLConnection)apiUrl.openConnection();
} catch (IOException e) {
throw new IOException(buildJSONError("Failed to open a connection.", "CONNECTION_FAILURE", ""));
}
// Set connection parameters for posting data
conn.setRequestMethod("POST");
conn.setUseCaches(false);
conn.setDoInput(true);
conn.setDoOutput(true);
// Write post data
try {
DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
wr.writeBytes(data);
wr.flush();
wr.close();
} catch (IOException e) {
throw new IOException(buildJSONError("Failed to post data in output stream (Connection OK?).", "POST_DATA_FAILURE", ""));
}
// Read the response from the server
InputStream is;
try {
is = conn.getInputStream();
} catch (IOException e) {
InputStream errStr = conn.getErrorStream();
if (errStr != null)
{
String errResponse = readInputStream(errStr);
throw new IOException(errResponse);
}
else
{
throw new IOException(buildJSONError("Failed to read error stream (Connection OK?).", "ERROR_STREAM_FAILURE", ""));
}
}
// Read and return response from the server
return readInputStream(is);
}
/* -------------------------------------
*
* Synchronous API calls
*
------------------------------------- */
public APIResponse<CustomerAuthentication> authenticateCustomer(HashMap<String,String> postData)
{
// Set the URL for this API call
String apiURL = m_apiDomain + "/customer/authenticate" + m_apiContentType;
Gson jsonConv = new Gson();
String apiResponse = "";
try
{
// Call the API action
apiResponse = callPost(apiURL, postData);
// Convert JSON response to the required object type
CustomerAuthentication customerAuth = jsonConv.fromJson(apiResponse, CustomerAuthentication.class);
// Build and return the API response object
APIResponse<CustomerAuthentication> result = new APIResponse<CustomerAuthentication>(true, customerAuth, null);
return result;
}
catch (IOException e)
{
// Build and return the API response object for a failure with error list
APIErrorList errorList = jsonConv.fromJson(e.getMessage(), APIErrorList.class);
APIResponse<CustomerAuthentication> result = new APIResponse<CustomerAuthentication>(false, null, errorList);
return result;
}
}
}
authenticateCustomer不能返回空指针。你是不是指在authenticateCustomer中发生异常，导致返回的APIResponse中的CustomerAuthentication为空？ - toto2