我在开发 reSTLet web 服务 + Android 应用程序时遇到了一个奇怪的问题。
当我尝试使用简单的获取从 Web 服务(本地部署在 Tomcat 7.0 上)恢复字符串时，如果来自服务器的字符串太大，Android 应用程序会崩溃并出现 NullPointerException。
基本上，方案如下
安卓应用
String retrievedString = clientResource.get();
网络服务器
return serializedString;
当 serializedString 有(不太)大量的字符(超过 2k 或类似的东西)时，retrievedString 得到一个 null 值.
我不知道这里的问题是什么，但我认为最有可能的原因是内存限制，由于 Android 堆、Java 堆、系统 RAM 甚至可能是 VirtualBox/Genymotion 限制。
您如何看待这个问题，我该如何解决？
编辑
根据要求，这里有更多的代码；我尽量避免不必要的东西和 try/catch block 。
服务器
ObjectMapper objectMapper = new ObjectMapper();
FilterProvider excludeThreadFilter =
new SimpleFilterProvider().addFilter("filter properties by name",
SimpleBeanPropertyFilter.serializeAllExcept("email", "password",
"threads", "messages", "subforumsSet", /*"parentSubforum",
"registeredDate", "threadSubforum"*/));
String st = objectMapper.writer(excludeThreadFilter).writeValueAsString(lt);
return st;
客户端
ClientResource cr = new ClientResource("...an ip address...");
ThreadsInSubforumIdByPage tis = cr.wrap(ThreadsInSubforumIdByPage.class);
// The line below is null
String retrievedString = tis.getThreads();
// The line below raise the NullPointerException
List<Thread> retrievedList =
DataHolder.getHolder().getObjectMapper().readValue(
retrievedString, new TypeReference<List<Thread>>(){});
异常
编辑:好的，我被 logcat 颜色误导了。我使用 Android Studio 才几天，我根本没有注意到 Log.e() 消息是以正常颜色书写的。
所以，这就是 logcat 报告的内容
5 .../W/System.err﹕ Unable to convert a [text/plain,UTF-8] representation into an object of class java.lang.String
5 .../W/System.err﹕ [ 175 3749: 3749 W/System.err ]
android.os.NetworkOnMainThreadException
5 .../W/System.err﹕ at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1117)
5 .../W/System.err﹕ at libcore.io.BlockGuardOs.recvfrom(BlockGuardOs.java:158)
5 .../W/System.err﹕ at libcore.io.IoBridge.recvfrom(IoBridge.java:525)
5 .../W/System.err﹕ at java.nio.SocketChannelImpl.readImpl(SocketChannelImpl.java:305)
5 .../W/System.err﹕ at java.nio.SocketChannelImpl.read(SocketChannelImpl.java:267)
5 .../W/System.err﹕ at org.restlet.engine.io.ReadableSocketChannel.read(ReadableSocketChannel.java:82)
5 .../W/System.err﹕ at org.restlet.engine.connector.Connection$1.read(Connection.java:232)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.fill(Buffer.java:395)
5 .../W/System.err﹕ at org.restlet.engine.io.ReadableBufferedChannel.onFill(ReadableBufferedChannel.java:157)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.process(Buffer.java:601)
5 .../W/System.err﹕ at org.restlet.engine.io.ReadableBufferedChannel.read(ReadableBufferedChannel.java:176)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.fill(Buffer.java:395)
5 .../W/System.err﹕ at org.restlet.engine.io.NbChannelInputStream.onFill(NbChannelInputStream.java:189)
5 .../W/System.err﹕ at org.restlet.engine.io.Buffer.process(Buffer.java:601)
5 .../W/System.err﹕ at org.restlet.engine.io.NbChannelInputStream.read(NbChannelInputStream.java:307)
5 .../W/System.err﹕ at java.io.InputStream.read(InputStream.java:163)
5 .../W/System.err﹕ at org.restlet.engine.io.BioUtils.copy(BioUtils.java:81)
5 .../W/System.err﹕ at org.restlet.engine.io.NioUtils.copy(NioUtils.java:148)
5 .../W/System.err﹕ at org.restlet.representation.ReadableRepresentation.write(ReadableRepresentation.java:104)
5 .../W/System.err﹕ at org.restlet.representation.ChannelRepresentation.write(ChannelRepresentation.java:76)
5 .../W/System.err﹕ at org.restlet.representation.ChannelRepresentation.write(ChannelRepresentation.java:82)
5 .../W/System.err﹕ at org.restlet.representation.Representation.getText(Representation.java:397)
9 .../W/System.err﹕ at org.restlet.engine.converter.DefaultConverter.toObject(DefaultConverter.java:260)
9 .../W/System.err﹕ at org.restlet.service.ConverterService.toObject(ConverterService.java:170)
9 .../W/System.err﹕ at org.restlet.resource.Resource.toObject(Resource.java:828)
9 .../W/System.err﹕ at org.restlet.engine.resource.ClientInvocationHandler.invoke(ClientInvocationHandler.java:240)
9 .../W/System.err﹕ at $Proxy23.getThreads(Native Method)
9 .../W/System.err﹕ at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.loadThreads(SubforumContentActivity.java:329)
9 .../W/System.err﹕ at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.access$400(SubforumContentActivity.java:283)
9 .../W/System.err﹕ at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground$1.run(SubforumContentActivity.java:301)
9 .../W/System.err﹕ at android.os.Handler.handleCallback(Handler.java:615)
9 .../W/System.err﹕ at android.os.Handler.dispatchMessage(Handler.java:92)
9 .../W/System.err﹕ at android.os.Looper.loop(Looper.java:137)
9 .../W/System.err﹕ at android.app.ActivityThread.main(ActivityThread.java:4745)
9 .../W/System.err﹕ at java.lang.reflect.Method.invokeNative(Native Method)
9 .../W/System.err﹕ at java.lang.reflect.Method.invoke(Method.java:511)
9 .../W/System.err﹕ at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
9 .../W/System.err﹕ at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
9 .../W/System.err﹕ at dalvik.system.NativeStart.main(Native Method)
[ 187 3749: 3749 E/class com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground NullPointerException
java.lang.NullPointerException
at com.fasterxml.jackson.core.JsonFactory.createParser(JsonFactory.java:822)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2153)
at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.loadThreads(SubforumContentActivity.java:331)
at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground.access$400(SubforumContentActivity.java:283)
at com.gmail.tigerjack89.pervasive.forum.SubforumContentActivity$LoadThreadsInBackground$1.run(SubforumContentActivity.java:301)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
在最初的问题中，我忽略了我正在使用 Jackson 来序列化我的实体，因为似乎只是间接地参与了这个问题。我的意思是，它仅在 retrievedString 为 null 时引发 NullPointerException。
AsyncTask代码
根据要求，这里有很多来 self 的 AsynkTask 的代码
private class LoadThreadsInBackground extends LoadFromServerInBackground
{
public LoadThreadsInBackground()
{
super(SubforumContentActivity.this);
}
@Override
protected Void doInBackground(final Boolean... params)
{
runOnUiThread(
new Runnable()
{
@Override
public void run()
{
currentThreadsPosition = threadsListView.getFirstVisiblePosition();
// Appending new data to threadsList
loadThreads(params[0]);
// Setting new scroll position
threadsListView.setSelectionFromTop(currentThreadsPosition + 1, 0);
}
});
return (null);
}
private void loadThreads(boolean reload)
{
if (reload) threadList.clear();
ClientResource cr = new ClientResource("IP address");
cr.setChallengeResponse(getAuthentication());
ThreadsInSubforumIdByPage tis = cr.wrap(ThreadsInSubforumIdByPage.class);
List<Thread> retrievedList;
String s = tis.getThreads();
retrievedList =
getObjectMapper().readValue(
s, new TypeReference<List<Thread>>(){});
threadList.addAll(retrievedList);
if ((retrievedList == null) || (retrievedList.size() < Constants
.RETRIEVED_PER_PAGE))
{
loadMoreThreadsButton.setEnabled(false);
if (retrievedList != null && retrievedList.size() == 0)
{ currentThreadsPage--; }
}
}
}
它扩展了这个抽象类
public abstract class LoadFromServerInBackground extends AsyncTask<Boolean, Void, Void>
{
private ProgressDialog mProgressDialog;
private Activity mInvokerActivity;
public LoadFromServerInBackground(Activity invokerActivity)
{
mInvokerActivity = invokerActivity;
}
@Override
protected void onPreExecute()
{
// Showing progress dialog before sending http request
mProgressDialog = new ProgressDialog(mInvokerActivity);
mProgressDialog.setMessage("Please wait ...");
mProgressDialog.setIndeterminate(true);
mProgressDialog.setCancelable(false);
mProgressDialog.show();
}
@Override
protected void onPostExecute(Void unused)
{
// closing progress dialog
mProgressDialog.dismiss();
}
}
最佳答案
您通过调用 runOnUiThread() 滥用了您的 AsyncTask。 runOnUiThread() 完全如其所说:在 UI 线程上执行 Runnable 中的代码。
Android 上的网络通信必须在后台线程上执行，runOnUiThread() 打破了这一要求。
试试这个版本的AsyncTask:
private class LoadThreadsInBackground extends LoadFromServerInBackground<List<Thread>> {
int currentThreadsPosition;
boolean reload;
public LoadThreadsInBackground() {
super(SubforumContentActivity.this);
}
@Override
protected void onPreExecute() {
super.onPreExecute();
currentThreadsPosition = threadsListView.getFirstVisiblePosition();
}
@Override
protected List<Thread> doInBackground(final Boolean... params) {
reload = params[0];
// Load data from network
return loadThreads();
}
@Override
protected void onPostExecute(List<Thread> retrievedList) {
super.onPostExecute(retrievedList);
if (reload) {
threadList.clear();
}
// Appending new data to threadsList
threadList.addAll(retrievedList);
if ((retrievedList == null) || (retrievedList.size() < Constants.RETRIEVED_PER_PAGE)) {
loadMoreThreadsButton.setEnabled(false);
if (retrievedList != null && retrievedList.size() == 0) {
currentThreadsPage--;
}
}
threadsListView.setSelectionFromTop(currentThreadsPosition + 1, 0);
}
private List<Thread> loadThreads() {
ClientResource cr = new ClientResource("IP address");
cr.setChallengeResponse(getAuthentication());
ThreadsInSubforumIdByPage tis = cr.wrap(ThreadsInSubforumIdByPage.class);
String s = tis.getThreads();
List<Thread> retrievedList = getObjectMapper().readValue(s, List<Thread>.class);
return retrievedList;
}
}
然后，让它扩展这个类:
public abstract class LoadFromServerInBackground<T> extends AsyncTask<Boolean, Void, T> {
private ProgressDialog mProgressDialog;
private Activity mInvokerActivity;
public LoadFromServerInBackground(Activity invokerActivity) {
mInvokerActivity = invokerActivity;
}
@Override
protected void onPreExecute() {
// Showing progress dialog before sending http request
mProgressDialog = new ProgressDialog(mInvokerActivity);
mProgressDialog.setMessage("Please wait ...");
mProgressDialog.setIndeterminate(true);
mProgressDialog.setCancelable(false);
mProgressDialog.show();
}
@Override
protected void onPostExecute(T unused) {
// closing progress dialog
mProgressDialog.dismiss();
}
}
关于java - 无法将 [text/plain,UTF-8] 表示形式转换为类 java.lang.String 的对象，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/28149169/