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Any straight forward way to convert javax.json.JsonObject to jakarta.json.JsonObject?

When starting the new Jakarta (9) project, I need to handle old (legacy libraries and modules) using the "javax" JSON-P API. Usually we end with this error:
java: incompatible types: jakarta.json.JsonObject cannot be converted
to javax.json.JsonObject
Unfortunately, the old code uses various javax.json.* classes, especially the JsonObjects, that are not usable in the new module's methods (as they are using jakarta.json.JsonObject).
We worked this around with producing the string out of the first JsonObject. Then we deserialized it into the other class instance, but is there any other more straightforward way to "retype" or convert the old javax JsonObject instances to new jakarta instances? And vice versa?

You are asking for a straightforward way - I think there is a common data format your code and the one you want to call understand: JSON
So the straightforward way would be to serialize your data, then parse it with the other parser. Short in lines of code, it may not be the fastest way to get the result.
Another option could be to walk the object class structure (it is a hierarchy), and with every step visited you construct a parallel object hierarchy in the other model.
Thy may be more lines of code but also has a chance to perform better.

Java `json_decode` (PHP) equivalent

I'm coming to Java from a PHP background, and am surprised to see that JSON to object conversion is so constrained. In all the Jackson tutorials I came across, it looks like the object to be read needs to be pre-defined. Thus, if my data is in, say, JSON API format, I need to write boilerplate code to strip out everything except the "data" part, and then somehow convert all the strings into objects one by one.
I really miss PHP's json_decode function, which will read any JSON and give you a PHP object to play with. It also builds the necessary structure into the object, adding arrays and sub-objects as needed. Of course I understand that Java is a compiled language, but I'm wondering how this can be made easier.

As a strongly typed language Java often has less of these "just give it to me"-type of functionalities, but that doesn't mean they don't exist. Even Jackson can deserialize JSON without a predefined schema, giving you Maps and Lists instead of domain objects.
Just remember that if you're working on "real" projects, there are plenty of advantages from having the schemas defined. They weren't invented to annoy you, but to make sure that you can trust your data being in the correct form (or find out early if it's not).

Differences between Java Serialization, JSON, JAXB?

Is an object's implementation of the Serializable interface in any way related to that object's ability to be serialized into JSON or XML?
Is there a name for the text format that Java serialization uses?
If not, should we not use the word "serialization" to describe exporting an object to JSON or XML, to avoid confusion?
At AO, what uses are typical for each of these three serialization methods?
I know that JAXB is usually used to convert XML to Java, not the other way around, but I heard that the reverse is possible too.

Serialization simply refers to exporting an object from a process-specific in-memory format to an inter-process format that can be read and understood by a different process or application. It may be text, or it may be binary, it doesn't matter. It's all serialization. The reverse processes (reading and parsing a serialized inter-process format into an in-memory, in-process format) is called deserialization.
In that sense, serializing an object into an ObjectStream is just as much serialization as serializing it to JSON or XML. ObjectStream serialization is very difficult to understand/parse by non-java (including humans. It is not "human-readable"), but is used because it can be done on pretty much any object without any special markup.
JSON/XML on the other hand require extra work to tell the parser how to map them to and from JSON/XML, but are very portable - pretty much every language can understand JSON/XML, including humans - it is "human-readable".

One purpose of serialization of Java objects is being able to write them to a (binary) file from which some Java program can read them back, getting the same objects into its memory. This usage is usually limited to Java applications, writing and reading, although some non-Java app might be written to understand the binary format.
Another frequently used serialization of Java objects is to write them to a text (or binary) file from which some (note the absence of: Java) program can read and reconstruct an object or data structure equivalent to the POJO. This, of course, also works in the reverse direction. (I'm adding "binary", because there are some binary formats not defined by Java that are architecture-independent, e.g., ASN.1.)
And, yes, JAXB works either way, but there are some difficulties if the XML is rather "outlandish", i.e., far away from what JAXB can handle or handle easily. But if you can design either the XML Schema or the Java classes, it works very well. JAXB being part of the JDK, you might prefer using it over other serializations if you need to go from Java to X or back. There are other languange binding for XML.

JSON, JS to Java and Vice Versa, and other languages

I want to use JSON to represent JS objects send them to a Java program, and vice versa.
I would also like to do this for other languages. C# to Java maybe?
What do I use to do this? (Not sure if it is refered to as serialization or data binding)
Edit:
Once a Java object is represented in JSON, does this mean that JavaScript can parse it and convert it to the corresponding JavaScript objects?

I would recommend using Gson for this. It has the advantage that it supports generics and nested beans very well and it is also fast enough. I've posted a Gson#fromJson() example which converts a fairly complex JSON string to Java here: Converting JSON to Java
The Gson#toJson() to convert any valid Javabean-like object to JSON is basically a piece of cake:
String json = new Gson().toJson(object);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
Edit: Once a Java object is represented in JSON, does this mean that JavaScript can parse it and convert it to the corresponding JavaScript objects?
Sure you can access them like a JS object. If you're new to using JSON in JS as well, then I can recomment this kickoff tutorial: http://www.hunlock.com/blogs/Mastering_JSON_%28_JavaScript_Object_Notation_%29

You could use Google Web Toolkit to share objects between javascript and Java. With GWT you write all your code in Java and then the GWT compiler will handle the serialization of the RPC calls from javascript to Java and vice vera.

If you mean over some sort connection (network, local pipe, etc), it would be called data serialization. You'd use a library to encode your objects. json.org has a list of libraries that can do what you want.
If you're writing a Java server with a JS front end, there's always GWT, too (I've never used, but heard great things about it)

To go from JSON to objects in Java, I've heard that json-simple works well. It maps the JSON to a Java Map, which can contain String, Numbers, Lists and other Maps. This is a little simpler than some other libraries, which map directly to Java objects that you need to create to represent the JSON.
For an exhaustive list of JSON libraries in most major languages including both Java and C#, check out json.org.

To serialize javascript objects for transmission to a server, I've used https://github.com/douglascrockford/JSON-js/blob/master/json2.js. Very nice and easy.
To get JSON data in and out of java, I found this library pretty straightforward:
http://json-lib.sourceforge.net/
Creating javascript objects from JSON is a non-issue. JSON is valid javascript. You can simply run eval on it, or use a javascript library, which may offer some built-in security, which eval doesn't have.

Unserialize in Java a serialized php object

Does anyone know if it is possible, actually if it has been done, to serialize an object in php and unserialize it in Java (java-php communication). Maybe an adapter will be needed.
What do you think?
Thanks

There is serialized-php-parser, which is a Java implementation that can parse php-serialized objects. In general, if you have the choice, I wouldn't recommend php-serialized as an exchange format, because it isn't ascii-safe (It contains null-bytes). Go with a format like xml or json instead. If you need a bit of type-information, xmlrpc is a good choice. It has good implementations for both php and Java.

PHP and Java both use their own (obviously different) serialization schemes. You could however use an interchange format both could read and write.
The two most obvious examples are XML and JSON.
There are others however such as Google Protocol Buffers.

Another Java project to work with the PHP serialization format is Pherialize.
Let's say you are serializing an array like this:
array(3) {
[0]=>
string(8) "A string"
[1]=>
int(12345)
[2]=>
bool(true)
}
Then you can unserialize it in Java with Pherialize like this:
MixedArray list = Pherialize.unserialize(data).toArray();
System.out.println("Item 1: " + list.getString(0));
System.out.println("Item 2: " + list.getInteger(1));
System.out.println("Item 3: " + list.getBoolean(2));

Theoretically, it's certainly possible. It's just bytes after all, and they can be parsed. Of course, the deserialized object would contain only data, not any of the PHP methods. If you want that, you'd have to rewrite the behaviour as Java classes that correspond directly with the PHP classes.
In practice, the main problem seems to be that the PHP serialization format does not seem to be formally specified - at least there is no link to a specification in the manual.
So you might have to dig through the code to understand the format.
All in all, it sounds like it would be much easier and more stable to use something like XML serialization - I'm sure both languages have libraries that faciliate this.

The JSON format would be a good place to start. There are implementations for Java, PHP and many other languages.
While initially based on the javascript object literal notation,
JSON proved convenient for lightweight data transfer between all types of systems.

add into pom.xml
<dependency>
<groupId>de.ailis.pherialize</groupId>
<artifactId>pherialize</artifactId>
<version>1.2.1</version>
</dependency>
then in code use
MixedArray list = Pherialize.unserialize(data).toArray(); // data is string `enter code here`

You can somehow make use of PHP's var_export() function for this, which returns a parseable string representation of the object you want to serialize.

I remember a snippet for Drupal (PHP CMS) where this functionality was needed. Just found it, so take a look at Serialized drupal node objects to java (should work with any PHP serialized object).
Maybe you can use that. I don't know whether there are issues with newer versions of PHP.

Serializing an object in PHP will dump the object properties. The resulting string isn't terribly complicated.
echo serialize(
array(1, null, "mystring", array("key"=>"value"))
);
Results in:
a:4:{i:0;i:1;i:1;N;i:2;s:8:"mystring";i:3;a:1:{s:3:"key";s:5:"value";}}
The string identifies datatypes, array lengths, array indexes and values, string lengths... Wouldn't take too much effort to reverse-engineer it and come up with your own parser, I think.

Like previous answers have mentioned, I would avoid PHP object serialization if possible. Use JSON (which is actually faster than serialize() in PHP), thrift or some other format that is more universal.
If you have no choice I have been working on a Jackson Module to enable reading and writing serialized PHP from Java. Jackson is a great JSON parser and since PHP serialization format is pretty similar it seemed like a good fit. It's not quite complete yet (writing is still a work in progress).

A better choice is to parse php serialized string to JSONArray, this repo (https://github.com/superalsrk/PhpSerialization) may help you

Note that there's a Java implementation of PHP. So you may be able to serialise the object and pass it to your Java-PHP instance, deserialise and then call into your Java infrastructure.
It all sounds a bit of an unholy mess, but perhaps worth looking at!

Use Web Services (REST, RPC, SOAP) or any other solution storing plain text that will allow you to read/rebuild the data from Java.

You may be also interested in using PHP/Java bridge (http://php-java-bridge.sourceforge.net/). It has own protocol. In their site said that it's fast implementation of bridge.

Try xstream (converts Java objects into readable XML) to serialize and then write your own PHP code to deserialize.