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Sonntag, 18. Oktober 2020

JSF: Keep data in Flash Scope on browser refresh and browser back

JSF supports different data scopes like the session scope to store data in the user session and the request scope to keep data for the lifespan of one request. Of course the goal should always be to keep the data in the most narrow scope possible because this approach will save server resources.

An interesting JSF scope is the flash scope. The flash scope expands the request scope to survive redirects. Redirects are an important part of the PRG-pattern which is commonly used in JSF applications. (for details on the PRG-pattern please see this post-redirect-get-and-jsf-20 blog post for details)

One problem I have encountered recently using the flash scope is that data is lost on a browser refresh and on a browser back. Some developers approach this problem by telling the users not to use this browser functionality (i.e. by java script checks) but in my opinion an application should support this basic functionality. Fortunately there is a suprisingly easy solution to this problem: By invoking the code:


JSF is instructed to keep the flash data (in this example the variable "context") even when the user hits F5 (refresh) or navigates back to a previous page with the browser's navigation buttons.

Mittwoch, 8. Juli 2020

Understanding Java Keystores for private key authentication

When your application needs to communicate over https (SSL) a KeyStore and a TrustStore may be involved.

The TrustStore usually holds the public keys of the servers that the client wants to establish a connection to. This store is located in the [jdk_home]\lib\security\cacerts file.

Usually it is sufficient to import a server certificate into this file in order to trust the issuing server with the help of the keytool command.

A KeyStore on the other hand usually holds private keys that can be used for authentication. The file is often in the PKCS12 format and has the file ending "pfx".

There are two ways to configure a TrustStore and a KeyStore. The easiest way is to use these JVM parameters:

 a) by configuration


 Obviously these variables need to be evaluated when connecting to a server. The apache http client (https://hc.apache.org/httpcomponents-client-5.0.x/index.html) and the jax-rs jersey client (https://mvnrepository.com/artifact/com.sun.jersey/jersey-client) do not read values from these variables. Therefore a different approach is necessary:

 b) programmatically load the TrustStore and KeyStore

This example uses the popular Apache HttpClient.
The following code shows how a SSL context is created with a truststore and a keystore which is needed when creating the client:

private SSLContext getSslContext() throws Exception {
    //load truststore (cacerts file)
    KeyStore serverKeystore = KeyStore.getInstance(KeyStore.getDefaultType());
    serverKeystore.load(new FileInputStream(trustStorePath), trustStorePassword.toCharArray());
    TrustManagerFactory serverTrustManager = TrustManagerFactory.getInstance("X509");
    //load keystore (pfx file)
    KeyStore userKeystore = KeyStore.getInstance("JKS");
    userKeystore.load(new FileInputStream(keystorePath), keyStorePassword.toCharArray());
    KeyManagerFactory userKeyFactory =
    userKeyFactory.init(userKeystore, keyStorePassword.toCharArray());
    SSLContext sslContext = SSLContext.getInstance("TLS");
    //init the SSL context with truststore and keystore
            serverTrustManager.getTrustManagers(), null);
    return sslContext;

This HttpClient is then created on basis of the SSLContext:

private CloseableHttpClient secureConnection() throws Exception {
    SSLContext sslContext = getSslContext();
    SSLConnectionSocketFactory sslConSocFactory = new SSLConnectionSocketFactory(sslContext);
    CloseableHttpClient httpclient = HttpClients.custom().setSSLSocketFactory(sslConSocFactory).build();
    return (httpclient);

Finally the client can be used to perform a https request with private key authentication:

private void performRequest(String url) throws Exception {
    CloseableHttpClient closeableHttpClient = secureConnection();
    HttpHead httpHead = new HttpHead(url);
    CloseableHttpResponse response = closeableHttpClient.execute(httpHead);

Sonntag, 29. März 2020

Obfuscating a jar file with yGuard and maven

yGuard is a nice tool that obfuscates java sources. Unfortunately there is no maven support.
It is possible to combine yGuard with maven but you have to make sure obfuscation is called in the correct step of the maven build process.

In order to run yGuard in a maven build process the following antrun plugin configuration in the maven pom.xml file of a jar artifact can be used:

    <taskdef name="yguard"
    <copy todir="target">
     <fileset dir="target" />
     <globmapper from="*.jar" to="backup/*_backup.jar" />
      <fileset dir="target" includes="myApp*.jar"/>
     <property name="naming-scheme" value="best"/>
     <rename logfile="renamelog.xml">
      <adjust replaceContent="true">
       <class methods="private" fields="private">
         <include name="de/myProject/unchanged/**/*"/>
       <class classes="none" methods="none" fields="none">
         <include name="de/myProject/obfuscate/**/*"/>
    <copy todir="target">
     <fileset dir="target" />
     <globmapper from="*_obf.jar" to="*.jar" />
     <fileset dir="target">
      <include name="*_obf.jar"/>
Note the following aspects:
  1. the antrun plugin is executed in the "integration-test" phase. During this phase the original jar file that needs to be obfuscated is already built and we have a chance to obfuscate it before it is installed into the local maven repo.
  2. before obfuscation a backup of the original jar file is done 
  3. after obfuscation (running the yguard-task) the obfuscated jar is renamed to its original name (with the help of copy and delete)
  4. Afterwards the obfuscated jar is installed into the local maven repository and can be referenced from other maven modules

Donnerstag, 5. Dezember 2019

Tagging an AWS Cloud Watch alarm

Tagging an AWS Cloud Watch Alarm

Recently tried to tag an alarm in AWS Cloud Watch. First I took a look at their REST-API documentation:

There are several things I found remarkable when trying to add a tag via the TagResource "action".

First of all there is no example included in the documentation. Not too bad because there is google with a lot of examples out there? Wrong, I couldn't find a single sample of someone doing such a REST call. After a lot of try and error I found a working solution:

The tag has to be included as a query parameter and a GET-request has to be issued in order to create a tag. (Please do not ask me why they are not implementing this as a post or put request like everyone else is doing). A key-value pair of tags has to be provided in this format as query parameters:


I am wondering how anyone is able to come up with this solution after reading the API documentation (see link above).

Since I found several other issues when implementing plain REST calls I continued with the Java SDK:

This API is well documented with a lot of examples and it is also easy to use. 

Conclusion: I think Amazon doesn't really want you to use their REST API directly. It is complicated, not well documented and has a strange architecture (GET-request to create data). Unfortunately I have found no hint that you are way better off using the SDKs that are available in multiple languages (C#, Go, JavaScript, Python, PHP, etc)

Freitag, 17. Mai 2019

Positioning of a primefaces dialog (p:dialog)

When using the primefaces dialog on a large page that has a vertical scrollbar, the dialog might not be visible because it is displayed at the top of the page and your current scroll position is too far down.

In order to make the dialog nicely centered on the visible part of the page I have used a small java script function that positions the dialog for me after it is rendered by primefaces:

function positionDialog(dialogId,anchorId) {
    var anchor = $(anchorId);
        "my": "center",
        "at": "center",
        "of": $(anchor)

In your xhtml page all you have to do is to define the dialog and the anchor to which the dialog has to be moved to. The anchor should be put on the place of the page where the dialog should appear:

<div id="myAnchorId"></div>
<p:dialog id="myId"
          header="My dialog"
          This is the content of my dialog

This way the dialog is always nicely centered no matter where your vertical scollbar is positioned at the moment.

Sonntag, 22. Oktober 2017

Autocompile im IntelliJ

Diese Woche bekam ich eine interessante Mail vom JRebel Produkt-Manager Sander Sõnajalg. Darin wird beschrieben, wie man im Intellij autocompile nutzen kann. Dies war bisher meines Wissens nicht möglich, man musste entweder Strg-F9 (Projekt bauen) oder Strg-Shift-F9 (aktuelle Klasse compilieren) nutzen, um Änderungen wirksam werden zu lassen.

Das Autocompile Feature lässt sich wie folgt nutzen:

  1. Im Intellij die Einstellungen öffnen, Compiler in den Filter eingeben, "Build project automatically" anwählen (obwohl hier die Warnung steht, dass dies nicht beim laufenden oder im Debug-Modus befindlichen Projekt funktioniert)
  2. Strg-Schift-A drücken im Intellij, "Registry..." in den Filter tippen, auswählen und "compiler.automake.allow.when.app.running" Checkbox aktivieren

Mit diesen Einstellungen werden Änderungen am Quelltext sofort im laufenden Programm wirksam, ohne dass eine zusätzliche Eingabe erfolgen muss. Die bisherigen Tests sahen sehr gut aus, sodass ich mich weiterhin über das geniale, wenn auch relativ teure JRebel freue. (gibt es Leute, die nach Nutzung von JRebel noch ohne dieses Tool leben können?)

Freitag, 6. Januar 2017

REST Service Exception Handling

Bei der Implementierung von REST-Services stellt sich der Entwickler früher oder später die Frage, wie eigentlich mit Exceptions umgegangen werden soll. Denn anders als bei der Implementierung von SOAP Services muss sich der Entwickler hier eine eigene Strategie überlegen, wie aufgetretene Fehler dem nutzenden System übermittelt werden sollen.

An dieser Stelle möchte ich eine Variante aus der Praxis vorstellen, die sich bewährt hat. Und zwar wird dazu die Klasse javax.ws.rs.ext.ExceptionMapper implementiert und mit der @Provider Annotation versehen. Eine weitergehende Konfiguration oder Aktivierung des Mappers ist nicht notwendig. Indem nun die Methode toResponse überschrieben wird, kann definiert werden, wie die Antwort des Servers im Falle einer Exception konkret aussehen soll.

Das Beispiel zeigt den Aufbau einer Nachricht bestehend aus einleitendem Text "An Error occured!", dem aktuellen Datum, dem Namen der Exception-Klasse und der Nachricht. Falls die Exception einen root cause hat, wird auch dieser noch mit ausgegeben.
Zusätzlich ist es sinnvoll, einen passenden HTTP-Status-Code mitzugeben, in diesem Fall Status Code 500 für "Internal Server Error", denn viele Clients fragen diesen Status-Code ab, um entsprechend reagieren zu können.

public class RestThrowableExceptionMapper implements ExceptionMapper<Throwable> {

    private HttpHeaders headers;

    public Response toResponse(Throwable throwable) {
        int status = Response.Status.INTERNAL_SERVER_ERROR.getStatusCode(); //defaults to internal server error 500;
        StringBuilder messageBuilder = new StringBuilder();
        messageBuilder.append("An Error occured! ");
        messageBuilder.append(new Date());
        messageBuilder.append(": ");
        messageBuilder.append("Cause -> ");
        messageBuilder.append(": ");
        // also append root cause of exception if present
        Throwable rootCause = ExceptionUtils.getRootCause(throwable);
        if (rootCause != null) {
            messageBuilder.append("; Root Cause -> ");
            messageBuilder.append(": ");
        return Response.status(status).