Testing the connection i.e. level4

src/main/java/io/jenkins/plugins/sample/global_configuration/OnboardingPluginGlobalConfiguration.java

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+package io.jenkins.plugins.sample.global_configuration;
+
+import hudson.Extension;
+import hudson.util.FormValidation;
+import hudson.util.Secret;
+import jenkins.model.GlobalConfiguration;
+import org.jenkinsci.Symbol;
+import org.kohsuke.stapler.QueryParameter;
+import org.kohsuke.stapler.verb.POST;
+
+import java.io.IOException;
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.util.Base64;
+
+
+@Extension
+@Symbol("OnboardingPlugin")
+public class OnboardingPluginGlobalConfiguration extends GlobalConfiguration {
+
+    private String name;
+    private String description;
+    private String url;
+    private String username;
+    private Secret password;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public String getDescription() {
+        return description;
+    }
+
+    public void setDescription(String description) {
+        this.description = description;
+    }
+
+    public String getUrl() {
+        return url;
+    }
+
+    public void setUrl(String url) {
+        this.url = url;
+    }
+
+    public String getUsername() {
+        return username;
+    }
+
+    public void setUsername(String username) {
+        this.username = username;
+    }
+
+    public Secret getPassword() {
+        return password;
+    }
+
+    public void setPassword(Secret password) {
+        this.password = password;
+        System.out.println("Password: " + password);
+    }
+
+    public FormValidation doCheckName(@QueryParameter String name) {
+        String regex = "^[a-zA-Z ]+$";
+        if (!name.matches(regex)) {
+            return FormValidation.warning("Name must contains characters & spaces");
+        }
+        return FormValidation.ok();
+    }
+
+    public FormValidation doCheckUsername(@QueryParameter String username) {
+        String regex = "^[a-zA-Z]+$";
+        if (!username.matches(regex)) {
+            return FormValidation.warning("Username must contains letters only");
+        }
+        return FormValidation.ok();
+    }
+
+    public FormValidation doCheckPwd(@QueryParameter String pwd) {
+        System.out.println("Check file system pwd:::::::: " + pwd);
+        return FormValidation.ok();
+    }
+
+    @POST
+    public FormValidation doTestConnection(@QueryParameter String url, @QueryParameter String username, @QueryParameter Secret password) throws IOException, InterruptedException {
+        String credentials = String.join(":", username, password.getPlainText());
+        String headerValue = "Basic " + Base64.getEncoder().encodeToString(credentials.getBytes());
+        var client = HttpClient.newHttpClient();
+
+
+        var request = HttpRequest.newBuilder().uri(URI.create(url))
+                .header("Authorization", headerValue)
+                .GET().build();
+
+        var responseFuture = client.send(request, HttpResponse.BodyHandlers.ofString());
+        if (responseFuture.statusCode() != 200) {
+            return FormValidation.error("Connection Failed: Provided configuration details are not correct. Response Code: "+ responseFuture.statusCode());
+        }
+        return FormValidation.ok("<>Connection Success!!! ");
+    }
+}

src/main/resources/io/jenkins/plugins/sample/global_configuration/OnboardingPluginGlobalConfiguration/config.jelly

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+<?jelly escape-by-default='true'?>
+<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form">
+    <f:section title="Onboarding Plugin">
+        <f:entry title="${%Name}" field="name">
+            <f:textbox checkMethod="post" />
+        </f:entry>
+        <f:entry title="${%Description}" field="description">
+            <f:textarea checkMethod="post" />
+        </f:entry>
+    </f:section>
+    <f:block>
+        <table>
+            <f:optionalBlock name="dynamic" title="Configuration for connection">
+                <f:entry title="${%URL}" field="url">
+                    <f:textbox />
+                </f:entry>
+                <f:entry title="${%Username}" field="username">
+                    <f:textbox  checkMethod="post" />
+                </f:entry>
+                <f:entry title="${%Password}" field="password">
+                    <f:password />
+                </f:entry>
+                <f:validateButton title="Test Connection" method="testConnection"
+                with="url,username,password" />
+            </f:optionalBlock>
+        </table>
+    </f:block>
+</j:jelly>