add checking connection function

as JabRef#6560 suggested

Author: chenyuheng

Diff for: src/main/java/org/jabref/gui/preferences/NetworkTab.fxml

@@ -53,5 +53,7 @@
                              GridPane.rowIndex="5"/>
         <Label fx:id="proxyAttentionLabel" styleClass="warning-message"
                text="%Attention: Password is stored in plain text!" GridPane.columnIndex="2" GridPane.rowIndex="5"/>
+        <Button fx:id="checkConnectionButton" text="%Check connection" onAction="#checkConnection"
+                prefWidth="200.0" GridPane.columnIndex="1" GridPane.rowIndex="6"/>
     </GridPane>
 </fx:root>

Diff for: src/main/java/org/jabref/gui/preferences/NetworkTabView.java

@@ -40,6 +40,7 @@ public class NetworkTabView extends AbstractPreferenceTabView<NetworkTabViewMode
     @FXML private Label proxyPasswordLabel;
     @FXML private CustomPasswordField proxyPassword;
     @FXML private Label proxyAttentionLabel;
+    @FXML private Button checkConnectionButton;
 
     private String proxyPasswordText = "";
     private int proxyPasswordCaretPosition = 0;
@@ -91,6 +92,8 @@ public void initialize() {
         proxyPassword.getRight().addEventFilter(MouseEvent.MOUSE_RELEASED, this::proxyPasswordMask);
         proxyPassword.getRight().addEventFilter(MouseEvent.MOUSE_EXITED, this::proxyPasswordMask);
 
+        checkConnectionButton.disableProperty().bind(proxyUse.selectedProperty().not());
+
         ActionFactory actionFactory = new ActionFactory(Globals.getKeyPrefs());
         actionFactory.configureIconButton(StandardActions.HELP, new HelpAction(HelpFile.REMOTE), remoteHelp);
 
@@ -120,4 +123,9 @@ private void proxyPasswordMask(MouseEvent event) {
             proxyPasswordCaretPosition = 0;
         }
     }
+
+    @FXML
+    void checkConnection() {
+        viewModel.checkConnection();
+    }
 }

Diff for: src/main/java/org/jabref/gui/preferences/NetworkTabViewModel.java

@@ -1,5 +1,6 @@
 package org.jabref.gui.preferences;
 
+import java.net.MalformedURLException;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
@@ -9,12 +10,19 @@
 import javafx.beans.property.SimpleStringProperty;
 import javafx.beans.property.StringProperty;
 
+import javafx.scene.control.ButtonType;
+import javafx.scene.control.DialogPane;
+import javafx.scene.control.Label;
+import javafx.scene.control.TextField;
+import javafx.scene.layout.GridPane;
+import kong.unirest.UnirestException;
 import org.jabref.Globals;
 import org.jabref.gui.DialogService;
 import org.jabref.gui.remote.JabRefMessageHandler;
 import org.jabref.logic.l10n.Localization;
 import org.jabref.logic.net.ProxyPreferences;
 import org.jabref.logic.net.ProxyRegisterer;
+import org.jabref.logic.net.URLDownload;
 import org.jabref.logic.remote.RemotePreferences;
 import org.jabref.logic.remote.RemoteUtil;
 import org.jabref.model.strings.StringUtil;
@@ -215,6 +223,52 @@ public boolean validateSettings() {
         return true;
     }
 
+    public void checkConnection() {
+        DialogPane dialogPane = new DialogPane();
+        GridPane settingsPane = new GridPane();
+        settingsPane.setHgap(4.0);
+        settingsPane.setVgap(4.0);
+
+        Label messageLabel = new Label();
+        messageLabel.setText(Localization.lang("Warning: your settings will be saved.\nEnter any URL to check connection to:"));
+
+        TextField url = new TextField();
+        url.setText("http://");
+
+        settingsPane.add(messageLabel, 1, 0);
+
+        settingsPane.add(url, 1, 1);
+        dialogPane.setContent(settingsPane);
+        String title = Localization.lang("Check Proxy Setting");
+        dialogService.showCustomDialogAndWait(
+                title,
+                dialogPane,
+                ButtonType.OK, ButtonType.CANCEL)
+                .ifPresent(btn -> {
+                            if (btn == ButtonType.OK) {
+                                String connectionProblemText = Localization.lang("Problem with connection: ");
+                                String connectionSuccessText = Localization.lang("Connection Successful!");
+                                storeProxySettings();
+                                System.out.println(url.getText());
+                                URLDownload dl;
+                                try {
+                                    dl = new URLDownload(url.getText());
+                                    int connectionStatus = dl.checkConnection();
+                                    if (connectionStatus == 200) {
+                                        dialogService.showInformationDialogAndWait(title, connectionSuccessText);
+                                    } else {
+                                        dialogService.showErrorDialogAndWait(title, connectionProblemText + Localization.lang("Request failed with status code ") + connectionStatus);
+                                    }
+                                } catch (MalformedURLException e) {
+                                    dialogService.showErrorDialogAndWait(title, connectionProblemText + e.getMessage());
+                                } catch (UnirestException e) {
+                                    dialogService.showErrorDialogAndWait(title, connectionProblemText + e.getMessage());
+                                }
+                            }
+                        }
+                );
+    }
+
     @Override
     public List<String> getRestartWarnings() {
         return restartWarning;

Diff for: src/main/java/org/jabref/logic/net/URLDownload.java

@@ -38,6 +38,7 @@
 import javax.net.ssl.TrustManager;
 import javax.net.ssl.X509TrustManager;
 
+import kong.unirest.UnirestException;
 import org.jabref.logic.util.io.FileUtil;
 import org.jabref.model.util.FileHelper;
 
@@ -171,6 +172,17 @@ public String getMimeType() {
         return "";
     }
 
+    public int checkConnection() {
+        Unirest.config().setDefaultHeader("User-Agent", "Mozilla/5.0 (Windows; U; WindowsNT 5.1; en-US; rv1.8.1.6) Gecko/20070725 Firefox/2.0.0.6");
+        // Try to use HEAD request to avoid downloading the whole file
+        try {
+            int statusCode = Unirest.head(source.toString()).asString().getStatus();
+            return statusCode;
+        } catch (UnirestException e){
+            throw e;
+        }
+    }
+
     public boolean isMimeType(String type) {
         String mime = getMimeType();