ConfigLib/README.md

# ConfigLib

**A Minecraft library for saving, loading, updating, and commenting YAML
configuration files.**

This library facilitates creating, saving, loading, updating, and commenting
YAML configuration files. It does so by automatically mapping instances of
configuration classes to serializable maps which are first transformed into YAML
and then saved to some specified file.

Information on how to [import](#import) this library can be found at the end of
this documentation. For a step-by-step tutorial that shows most features of this
library in action check out
the [Tutorial](https://github.com/Exlll/ConfigLib/wiki/Tutorial) page on the
wiki!

## Features

* Automatic creation, saving, loading, and updating of configuration files
* Support for comments through annotations
* Support for all primitive types, their wrapper types, and strings
* Support for all Java enums, records, and POJOs (+ inheritance!)
* Support for (nested) lists, sets, arrays, and maps
* Support for `BigInteger` and `BigDecimal`
* Support for `LocalDate`, `LocalTime`, `LocalDateTime`, and `Instant`
* Support for `UUID`, `File`, `Path`, `URL`, and `URI`
* Support for Bukkit's `ConfigurationSerializable` types (e.g. `ItemStack`)
* Option to format the names of configuration elements
* Option to exclude fields from being converted
* Option to customize null handling
* Option to customize serialization by providing your own serializers
* Option to add headers and footers to configuration files
* ...and a few more!

## Usage example

This section contains a short usage example to get you started. The whole range
of features is discussed in the following sections. Information on how
to [import](#import) this library is located at the end of this documentation.

For a step-by-step tutorial with a more advanced example check out
the [Tutorial](https://github.com/Exlll/ConfigLib/wiki/Tutorial) page on the
wiki.

If you want support for Bukkit classes like `ItemStack`, check out
the [Configuration properties](#configuration-properties) section.

```java
public final class Example {
    // * To create a configuration annotate a class with @Configuration and make sure that
    //   it has a no-args constructor.
    // * Now add fields to that class and assign them default values.
    // * That's it! Fields can be private; setters are not required.
    @Configuration
    public static class BaseConfiguration {
        private String host = "127.0.0.1";
        private int port = 1234;
        // The library supports lists, sets, and maps.
        private Set<String> blockedAddresses = Set.of("8.8.8.8");
        // Fields can be ignored by making them final, transient, static or by
        // annotating them with @Ignore.
        private final double ignoreMe = 3.14;
    }

    // This library supports records; no @Configuration annotation required
    public record User(
            String username,
            @Comment("Please choose a strong password.")
            String password
    ) {}

    // Subclassing of configurations and nesting of configurations in other configurations
    // is also supported. Subclasses don't need to be annotated again.
    public static final class UserConfiguration extends BaseConfiguration {
        // You can add comments with the @Comment annotation. Each string in the comment
        // array is written (as a comment) on a new line.
        @Comment({"The admin user has full access.", "Choose a proper password!"})
        User admin = new User("root", "toor"); // The User class is a record!
        List<User> blockedUsers = List.of(
                new User("user1", null), // null values are supported
                new User("user2", null)
        );
    }

    public static void main(String[] args) {
        var configFile = Paths.get("/tmp/config.yml");
        var config = new UserConfiguration();

        // Save an instance to the configuration file
        YamlConfigurations.save(configFile, UserConfiguration.class, config);

        // Load a new instance from the configuration file
        config = YamlConfigurations.load(configFile, UserConfiguration.class);
        System.out.println(config.admin.username);
        System.out.println(config.blockedUsers);

        // Modify the configuration and save it again
        config.blockedUsers.add(new User("user3", "pass3"));
        YamlConfigurations.save(configFile, UserConfiguration.class, config);
    }
}
```

By running the above code, a new YAML configuration is created
at `/tmp/config.yml`. Its content looks like this:

```yaml
host: 127.0.0.1
port: 1234
blockedAddresses:
  - 8.8.8.8
# The admin user has full access.
# Choose a proper password!
admin:
  username: root
  # Please choose a strong password.
  password: toor
blockedUsers:
  - username: user1
  - username: user2
  - username: user3
    password: pass3
```

Two things are noticeable here:

1. Not every user in the `blockedUsers` list has a `password` mapping. This is
   because null values are not output by default. That behavior can be changed
   by the builder.
2. The password of the user with username `user3` has no comment. This is due to
   limitations of the YAML library. Configurations in lists, sets, or maps
   cannot have their comments printed.

## General information

In the following sections the term _configuration type_ refers to any Java
record type or to any non-generic class that is directly or indirectly (i.e.
through subclassing) annotated with`@de.exlll.configlib.Configuration`.
Accordingly, the term _configuration_ refers to an instance of such a type. A
_configuration element_ is either a class field or a record component of a
configuration type.

### Declaring configuration types

To declare a configuration type, either define a Java record or annotate a class
with `@Configuration` and make sure that it has a no-args constructor. The
no-args constructor can be `private`. Inner classes (i.e. the ones that are
nested but not `static`) have an implicit synthetic constructor with at least
one argument and are, therefore, not supported.

Now simply add components to your record or fields to your class whose type is
any of the supported types listed in the next section. You can (and should)
initialize all fields of a configuration class with non-null default values.

### Supported types

A configuration type may only contain configuration elements of the following
types:

| Type class                  | Types                                                              |
|-----------------------------|--------------------------------------------------------------------|
| Boolean types               | `boolean`, and `Boolean`                                           |
| Integer types               | `byte`, `short`, `int`, `long`, and their respective wrapper types |
| Floating point types        | `float`, `double`, and their respective wrapper types              |
| Characters and strings      | `char`, `Character`, `String`                                      |
| Big numeric types           | `BigInteger`, `BigDecimal`                                         |
| Time related types          | `LocalTime`, `LocalDate`, `LocalDateTime`, `Instant`               |
| Utility types               | `UUID`, `File`, `Path`, `URL`, `URI`                               |
| Enums                       | Any Java enum                                                      |
| Configurations              | Any Java record or any class annotated with `@Configuration`       |
| `ConfigurationSerializable` | All Bukkit classes that implement this interface, like `ItemStack` |
| Collections                 | (Nested) Lists, sets, maps*, or arrays of previously listed types  |

(*) Map keys can only be of simple or enum type, i.e. they cannot be in
the `Collections`, `Configurations`, or `ConfigurationSerializable` type class.

For all types that are not listed in the table above, you can provide your
own [custom serializer](#custom-serializers).

#### Examples of supported types

The following class contains examples of types that this library supports:

```java
public final class SupportedTypes {
    boolean supported;
    Character supported;
    String supported;
    LocalTime supported;
    UUID supported;
    ExampleEnum supported;    // where 'ExampleEnum' is some Java enum type
    ExampleConfig supported;  // where 'ExampleConfig' is a class annotated with @Configuration
    ExampleRecord supported;  // where 'ExampleRecord' is a Java record

    /* collection types */
    List<BigInteger> supported;
    Set<Double> supported;
    LocalDate[] supported;
    Map<ExampleEnum, ExampleConfig> supported;

    /* nested collection types */
    List<Map<ExampleEnum, LocalDate>> supported;
    int[][] supported;
    Map<Integer, List<Map<Short, Set<ExampleRecord>>>> supported;

    // supported if a custom serializer is registered
    java.awt.Point supported;

    // supported when a special properties object is used (explained further below)
    org.bukkit.inventory.ItemStack supported;
}
```

<details>
 <summary>Examples of unsupported types</summary>

The following class contains examples of types that this library does (and will)
not support:

```java
public final class UnsupportedTypes<T> {
    Map<Point, String> unsupported;        // invalid map key
    Map<List<String>, String> unsupported; // invalid map key
    Box<String> unsupported;               // custom parameterized type
    List<? extends String> unsupported;    // wildcard type
    List<?> unsupported;                   // wildcard type
    List<?>[] unsupported;                 // wildcard type
    T unsupported;                         // type variable
    List unsupported;                      // raw type
    List[] unsupported;                    // raw type
    List<String>[] unsupported;            // generic array type
    Set<Integer>[] unsupported;            // generic array type
    Map<Byte, Byte>[] unsupported;         // generic array type
}
```

**NOTE:** Even though this library does not support these types, it is still
possible to serialize them by providing a custom serializer via
the [`@SerializeWith`](#the-serializewith-annotation) annotation. That
serializer then has to be applied to top-level type (i.e. `nesting` must be set
to `0`, which is the default).

</details>

### Loading and saving configurations

There are two ways to load and save configurations. Which way you choose depends
on your liking. Both ways have five methods in common:

* The `save` method converts a configuration to a string in YAML format and
  saves that string to a file. The file is created if it does not exist and is
  overwritten otherwise.
* The `load` method creates a new configuration instance and populates it with
  values taken from a file. For classes, the no-args constructor is used to
  create a new instance. For records, the canonical constructor is called.
* The `update` method is a combination of `load` and `save` and the method you'd
  usually want to use: it takes care of creating the configuration file if it
  does not exist and otherwise updates it to reflect changes to (the
  configuration elements of) the configuration type.
* The `write` method works the same way as the `save` method but writes the
  string to a `java.io.OutputStream`.
* The `read` method works the same way as the `load` method but reads the values
  from a `java.io.InputStream`.

<details>
 <summary>Example of <code>update</code> behavior when configuration file exists</summary>

Let's say you have the following configuration type

```java 
@Configuration 
public final class C {
    int i = 10; 
    int j = 11; 
}
```

... and a YAML configuration file that contains:

```yaml
i: 20
k: 30
```

Now, when you call the `update` method for that configuration type and file
using any of the two options listed below, the configuration instance
that `update` returns will have its `i` variable initialized to `20` and its `j`
variable will have its default of `11`. After the operation, the configuration
file will contain the following content (note that `k` has been dropped):

```yaml
i: 20
j: 11
```

<hr>

</details>

To exemplify the usage of these five methods we assume for the following
sections that you have implemented the configuration type below and have access
to some regular `java.nio.file.Path` object `configurationFile`.

```java 
@Configuration
public final class Config { /* some fields */ }
```

#### Option 1

The first option is to create a `YamlConfigurationStore` instance and use it to
save, load, or update configurations.

```java 
YamlConfigurationProperties properties = YamlConfigurationProperties.newBuilder().build();
YamlConfigurationStore<Config> store = new YamlConfigurationStore<>(Config.class, properties);

Config config1 = store.load(configurationFile);
store.save(config1, configurationFile);
Config config2 = store.update(configurationFile);
```

Using a `YamlConfigurationStore` directly is always more efficient than the
second option show below, especially if you are calling any of its method
multiple times.

#### Option 2

The second option is to use the static methods from the `YamlConfigurations`
class.

```java 
Config config1 = YamlConfigurations.load(configurationFile, Config.class);
YamlConfigurations.save(configurationFile, Config.class, config1);
Config config2 = YamlConfigurations.update(configurationFile, Config.class);
```

Each of these methods has two additional overloads: One that takes a properties
object and another that lets you configure a properties object builder. For
example, the overloads of the `load` method are:

```java 
// overload 1
YamlConfigurationProperties properties = YamlConfigurationProperties.newBuilder().build();
Config config1 = YamlConfigurations.load(configurationFile, Config.class, properties);

// overload 2
Config config2 = YamlConfigurations.load(
    configurationFile,
    Config.class,
    builder -> builder.inputNulls(true).outputNulls(false)
); 
```

<hr>

All five methods can also be passed a Java record instead of a class. To provide
default values for records when calling the `update` method, you can add a
constructor with no parameters that initializes its components. This constructor
is only called if the configuration file does not exist.

```java 
record User(String name, String email) {
    User() { this("John Doe", "john@doe.com"); }
}
User user = YamlConfigurations.update(configurationFile, User.class);
```

### Configuration properties

Instances of the `ConfigurationProperties` class allow customization of how
configurations are stored and loaded. To create such an instance, instantiate a
new builder using the `YamlConfigurationProperties.newBuilder()` method,
configure it, and finally call its `build()` method. Alternatively, you can use
the `toBuilder()` method of an existing `YamlConfigurationProperties` to create
a new builder that is initialized with values takes from the properties object.

Check out the methods of the builder class to see which configuration options
are available.

#### Support for Bukkit classes like `ItemStack`

There is a special `YamlConfigurationProperties` object with
name `BUKKIT_DEFAULT_PROPERTIES` that adds support for
Bukkit's `ConfigurationSerializable` types. If you want to use any of these
types in your configuration, you have to use that object as a starting point:

```java 
YamlConfigurationProperties properties = ConfigLib.BUKKIT_DEFAULT_PROPERTIES.toBuilder()
     // ...further configure the builder...
    .build();
```

To get access to this object, you have to import `configlib-paper` instead
of `configlib-yaml` as described in the [Import](#import) section.

### Comments

The configuration elements of a configuration type can be annotated with
the `@Comment` annotation. This annotation takes an array of strings. Each of
these strings is written onto a new line as a comment. The strings can
contain `\n` characters. Empty strings are written as newlines (not as
comments).

If a configuration type _C_ that defines comments is used (as a configuration
element) within another configuration type, the comments of _C_ are written with
the proper indentation. However, if instances of _C_ are stored inside a
collection, their comments are not printed when the collection is written.

Serializing the following configuration as YAML ...

```java 
@Configuration
public final class ExampleConfiguration {
    @Comment({"Hello", "", " ", "World"})
    private String commentedField = "commented field";
}
```

... results in the YAML file shown below:

```yaml
 # Hello

 #  
 # World
 commentedField: commented field
```

Similarly, if you define the following record configuration and save it ...

```java 
record Address(@Comment("The street") String street) {}
record User(@Comment("The name") String name, @Comment("The address") Address address) {}

User user = new User("John Doe", new Address("10 Downing St"));
```

... you get:

```yaml
# The name
name: John Doe
# The address
address:
  # The street
  street: 10 Downing St
```

### Subclassing

Subclassing of configurations types is supported. Subclasses of configuration
types don't need to be annotated with `@Configuration`. When a configuration is
written, the fields of parent classes are written before the fields of the child
in a top to bottom manner. Parent configurations can be `abstract`.

#### Shadowing of fields

Shadowing of fields refers to the situation where a subclass of configuration
has a field that has the same name as a field in one of its super classes.
Shadowing of fields is currently not supported. (This restriction might easily
be lifted. If you need this feature, please open an issue and describe how to
handle name clashes.)

### Ignoring and filtering fields

Fields that are `final`, `static`, `transient` or annotated with `@Ignore` are
neither serialized nor updated during deserialization. You can filter out
additional fields by providing an instance of `FieldFilter` to the configuration
properties. Record components cannot be filtered.

### Handling of missing and `null` values

#### Missing values

When a configuration file is read, values that correspond to a configuration
element might be missing. That can happen, for example, when somebody deleted
that value from the configuration file, when you add configuration elements to
your configuration type, or when the `NameFormatter` that was used to create
that file is replaced.

In such cases, fields of configuration classes keep the default value you
assigned to them and record components are initialized with the default value of
their corresponding type.

#### Null values

**NOTE:** Null values written to a configuration file generally don't give any
indication about which kinds of values the configuration expects. Therefore,
they not only make it harder for the users of that configuration file to
properly configure it, but they might also prevent loading a configuration if
the values the users set are of the wrong type.

Although strongly discouraged, null values are supported
and `ConfigurationProperties` let you configure how they are handled when
serializing and deserializing a configuration:

* By setting `outputNulls` to false, configuration elements, and collection
  elements that are null are not output. Any comments that belong to such fields
  are also not written.
* By setting `inputNulls` to false, null values read from the configuration file
  are treated as missing and are, therefore, handled as described in the section
  above.
* By setting `inputNulls` to true, null values read from the configuration file
  override the corresponding default values of a configuration class with null
  or set the component value of a record type to null. If the configuration
  element type is primitive, an exception is thrown.

The following code forbids null values to be output but allows null values to be
input. By default, both are forbidden which makes the call to `outputNulls` in
this case redundant.

```java 
YamlConfigurationProperties.newBuilder()
        .outputNulls(false)
        .inputNulls(true)
        .build();
```

### Formatting the names of configuration elements

You can define how the names of configuration elements are formatted by
configuring the configuration properties with a custom formatter. Formatters are
implementations of the `NameFormatter` interface. You can implement this
interface yourself or use one of the several formatters this library provides.
These pre-defined formatters can be found in the `NameFormatters` class.

The following code formats fields using the `IDENTITY` formatter (which is the
default).

```java 
YamlConfigurationProperties.newBuilder()
        .setNameFormatter(NameFormatters.IDENTITY)
        .build();
```

### Type conversion and serializer selection

Before instances of the types listed in the [supported types](#supported-types)
section can be stored, they need to be converted into serializable types (i.e.
into types the underlying YAML library knows how to handle). The conversion
happens according to the following table:

| Source type                 | Target type      |
|-----------------------------|------------------|
| Boolean types               | `Boolean`        |
| Integer types               | `Long`           |
| Floating point types        | `Double`         |
| Characters and strings      | `String`         |
| Big numeric types           | `String`         |
| Time related types          | `String`         |
| Utility types               | `String`         |
| Enums                       | `String`         |
| Configurations              | `Map<String, ?>` |
| `Set<S>`                    | `List<T>`        |
| `List<S>`                   | `List<T>`        |
| `S[]`                       | `List<T>`        |
| `Map<S1, S2>`               | `Map<T1, T2>`    |
| `ConfigurationSerializable` | `String`         |

#### Serializer selection

To convert the value of a configuration element `E` with (source) type `S` into
a serializable value of some target type, a serializer has to be selected.
Serializers are instances of the `de.exlll.configlib.Serializer` interface and
are selected based on `S`. Put differently, serializers are, by default, always
selected based on the compile-time type of `E` and never on the runtime type of
its value.

<details>
 <summary>Why should I care about this?</summary>

This distinction makes a difference (and might lead to confusion) when you have
configuration elements that are configuration classes, and you extend those
classes. Concretely, assume you have written two configuration classes `A`
and `B` where `B extends A`. Then, if you use `A a = new B()` in your main
configuration, only the fields of a `A` will be stored when you save your main
configuration. That is because the serializer of field `a` was selected based on
the compile-time type of `a` which is `A` and not `B`. The same happens if you
have a `List<A>` and put instances of `B` (or some other subclass of `A`) in it.

If you need such behavior, have a look at
the [`@Polymorphic`](#the-polymorphic-annotation) annotation.

</details>

<details>
 <summary>Order of serializer selection</summary>

You can override the default selection by annotating a configuration
element with [`@SerializeWith`](#the-serializewith-annotation), by annotating a
type with `@SerializeWith`, or by adding your own serializer for `S` to the
configuration properties. When you do so, it can happen that there multiple
serializers available for a particular configuration element and its type. In
that case, one of them chosen according to the following precedence rules:

1. If the element is annotated with `@SerializeWith` and the `nesting` matches,
   the serializer referenced by the annotation is selected.
2. Otherwise, if the `ConfigurationProperties` contain a serializer for the type
   in question, that serializer is returned.
    * Serializers created by factories that were added
      through `addSerializerFactory` for some type take precedence over
      serializers added by `addSerializer` for the same type.
3. If the type is annotated `@SerializeWith`, the serializer referenced by the
   annotation is selected.
4. If the type is annotated with an annotation which is annotated
   with `@SerializeWith`, the serializer referenced by `@SerializeWith` is
   returned.
5. If this library defines a serializer for that type, that serializer is
   selected.
6. Ultimately, if no serializer can be found, an exception is thrown.

For lists, sets, and maps, the algorithm is applied to their generic type
arguments recursively first.

</details>

##### The `@SerializeWith` annotation

The `@SerializeWith` annotation enforces the use of the specified serializer for
a configuration element or type. It can be applied to configuration elements
(i.e. class fields and record components), to types, and to other annotations.

```java 
@SerializeWith(serializer = MyPointSerializer.class)
Point point;
```

```java 
@SerializeWith(serializer = SomeClassSerializer.class)
public final class SomeClass {/* ... */} 
```

The serializer referenced by this annotation is selected regardless of whether
the annotated type or type of configuration element matches the type the
serializer expects.

If the annotation is applied to a configuration element and that element is an
array, list, set, or map, a nesting level can be set to apply the serializer not
to the top-level type but to its elements. For maps, the serializer is applied
to the values and not the keys.

```java 
@SerializeWith(serializer = MySetSerializer.class, nesting = 1)
List<Set<String>> list;
```

Setting `nesting` to an invalid value, i.e. a negative one or one that is
greater than the number of levels the element actually has, results in the
serializer not being selected. For type annotations, the `nesting` has no
effect.

<details>
 <summary>More <code>nesting</code> examples</summary>

In this example...

```java 
@SerializeWith(serializer = MySetSerializer.class, nesting = 1)
List<Set<String>> list;
```

* a nesting of `0` would apply the serializer to `list` (which is of
  type `List<Set<String>>`),
* a nesting of `1` would apply it to the `Set<String>` elements within `list`,
  and
* a nesting of `2` would apply it to the strings within the sets of `list`.

However, since the referenced serializer `MySetSerializer` most likely
expects `Set`s as input, setting `nesting` to `0` or `2` would result in an
exception being thrown when the configuration is serialized.

Some more examples:

```java 
// MyListSerializer is applied to 'list'
@SerializeWith(serializer = MyListSerializer.class)
List<Set<String>> list;

// MySetSerializer is applied to the Set<String> elements of 'list'
@SerializeWith(serializer = MySetSerializer.class, nesting = 1)
List<Set<String>> list;

// MyStringSerializer is applied to the strings within the set elements of 'list'
@SerializeWith(serializer = MyStringSerializer.class, nesting = 2)
List<Set<String>> list;

// MyMap0Serializer is applied to 'map'
@SerializeWith(serializer = MyMap0Serializer.class)
Map<Integer, Map<String, Double>> map;

// MyMap1Serializer is applied to the Map<String, Double> values of 'map'
@SerializeWith(serializer = MyMap1Serializer.class, nesting = 1)
Map<Integer, Map<String, Double>> map;

// MyDoubleSerializer is applied to the doubles within the nested values of 'map'
@SerializeWith(serializer = MyDoubleSerializer.class, nesting = 2)
Map<Integer, Map<String, Double>> map; 
```

</details>

#### The `@Polymorphic` annotation

The `@Polymorphic` annotation indicates that the annotated type is polymorphic.
Serializers for polymorphic types are not selected based on the compile-time
types of configuration elements, but instead are chosen at runtime based on the
actual types of their values.

This enables adding instances of subclasses / implementations of a polymorphic
type to collections. The subtypes must be valid configurations.

```java 
@Polymorphic
@Configuration
static abstract class A { ... }

static final class Impl1 extends A { ... }
static final class Impl2 extends A { ... }
    
List<A> as = List.of(new Impl1(...), new Impl2(...), ...); 
```

For correct deserialization, if an instance of polymorphic type (or one of its
implementations / subclasses) is serialized, an additional property that holds
type information is added to its serialization. By default, that type
information is the Java class name of the actual type. It is possible to provide
type aliases by using the `PolymorphicTypes` annotation.

```java 
@Polymorphic
@PolymorphicTypes({
        @PolymorphicTypes.Type(type = Impl1.class, alias = "IMPL_1"),
        @PolymorphicTypes.Type(type = Impl2.class, alias = "IMPL_2")
})
interface B { ... }

record Impl1(...) implements B { ... }
record Impl2(...) implements B { ... }
```

### Custom serializers

If you want to add support for a type that is not a Java record or whose class
is not annotated with `@Configuration`, or if you don't like how one of the
supported types is serialized by default, you can write your own custom
serializer.

Serializers are instances of the `de.exlll.configlib.Serializer` interface. When
implementing that interface you have to make sure that you convert your source
type into one of the valid target types listed
in [type conversion](#type-conversion-and-serializer-selection) section.

The serializer then has to be registered through a `ConfigurationProperties`
object or alternatively be applied to a configuration element or type
with [`@SerializeWith`](#the-serializewith-annotation). If you want to use
the `@SerializeWith` annotation, your serializer class must either have a
constructor with no parameters or one with exactly one parameter of
type [`SerializerContext`](#the-serializercontext-interface).

The following `Serializer` serializes instances of `java.awt.Point` into strings
and vice versa.

```java
public final class PointSerializer implements Serializer<Point, String> {
    @Override
    public String serialize(Point element) {
        return element.x + ":" + element.y;
    }

    @Override
    public Point deserialize(String element) {
        String[] parts = element.split(":");
        int x = Integer.parseInt(parts[0]);
        int y = Integer.parseInt(parts[1]);
        return new Point(x, y);
    }
}
```

```java 
YamlConfigurationProperties properties = YamlConfigurationProperties.newBuilder()
        .addSerializer(Point.class, new PointSerializer())
        .build(); 
```

##### The `SerializerContext` interface

Instances of the `SerializerContext` interface contain contextual information
for custom serializers. A context object gives access to the configuration
properties, configuration element, and the annotated type for which the
serializer was selected.

Context objects can be obtained when adding serializer factories through
the `addSerializerFactory` method:

```java
public final class PointSerializer implements Serializer<Point, String> {
    private final SerializerContext context;

    public PointSerializer(SerializerContext context) {
        this.context = context;
    }
    // implementation ...
}
```

```java 
YamlConfigurationProperties properties = YamlConfigurationProperties.newBuilder()
        .addSerializerFactory(Point.class, PointSerializer::new)
        .build();
```

Custom serializers used with `@SerializeWith` are allowed to declare a
constructor with one parameter of type `SerializerContext`. If such a
constructor exists, a context object is passed to it when the serializer is
instantiated by this library.

### Post-processing

There are two ways to apply some post-processing to your configurations:

- The first is to annotate a method in your configuration type with the
  `@PostProcess` annotation.
- The second is to add post-processor functions to a `ConfigurationProperties`
  object. These functions are then applied to some set of configuration elements
  that is defined by a `ConfigurationElementFilter`.

Both ways of post-processing can be applied at the same time. In this case,
the post-processor functions added to a `ConfigurationProperties` object run
first.

#### Post-process configurations via annotated method

One way to apply post-processing to your configuration is to annotate some
method of your configuration type with the `@PostProcess` annotation.

```java
@Configuration
public final class Config {
    private int i = 10;
    private String s = "abc";

    @PostProcess
    private void postProcess() {
        this.i = this.i * 2;
        this.s = this.s.repeat(2);
    }
}
```

The return type of the `@PostProcess` method must either be `void` or the same
type as the type in which that method is defined. In the first case, the method
is simply executed. In the latter case, the return value of the method replaces
the current instance when initializing a configuration. This is, in particular,
useful for Java records whose fields are final and cannot be modified.

```java
public record Config(int i, String s) {
    @PostProcess
    private Config postProcess() {
        return new Config(i * 2, s.repeat(2));
    }
}
```

The name of the `@PostProcess` method can be any valid Java method name.
However, your configuration type is allowed to define at most one such method
and `@PostProcess` methods of parent classes are _not_ executed.

#### Post-process configuration elements by condition

The second way to apply post-processing to your configuration is to define
a `ConfigurationElementFilter`. Such a filter implicitly defines a set of
configuration elements to which some post-processing function should be applied.
Both, filters and post-processing functions, can be added via
the `ConfigurationProperties#addPostProcessor` method at the same time and the
function is then applied to all configuration elements that are defined by the
filter.

For example, to double the values of _all_ configuration elements of type `int`,
you would add the following filter and post-processing function:

```java
ConfigurationProperties.newBuilder()
        .addPostProcessor(
                // Predicate<? super ConfigurationElement<?>> filter
                element -> element.type().equals(int.class),
                // UnaryOperator<?> postProcessor
                (Integer value) -> value * 2
        )
        .build();
```

Note that it is your responsibility to make sure that the filter only selects
configuration elements whose type matches the type the post-processing function
expects.

Also note, that the post-processing function will be applied regardless of
whether a configuration file contained a value for some specific element.
This means that your post-processing function should properly handle `null`
input values if, for example, you allow the input of such values.

The `ConfigurationElementFilter` interface defines static factories to
facilitate the creation of common filters:

```java
ConfigurationElementFilter.byType(Class<?> type)
ConfigurationElementFilter.byPostProcessKey(String key)
```

The second factory creates a filter that selects all configuration elements that
are annotated with `@PostProcess` and where the `key()` method of that
annotation returns the given `key`.

In the following example, the values of `a` and `b` are doubled, the value
of `c` is tripled, `d` is set to zero, and no post-processing is applied
to `e` and `f`.

```java
record Config(
        @PostProcess(key = "double") int a,
        @PostProcess(key = "double") int b,
        @PostProcess(key = "tripple") int c,
        @PostProcess int d,
        @PostProcess(key = "missing processor") int e,
        int f
) {}

ConfigurationProperties.newBuilder()
        .addPostProcessor(
                ConfigurationElementFilter.byPostProcessKey("double"),
                (Integer value) -> value * 2
        )
        .addPostProcessor(
                ConfigurationElementFilter.byPostProcessKey("tripple"),
                (Integer value) -> value * 3
        )
        .addPostProcessor(
                ConfigurationElementFilter.byPostProcessKey(""),
                (Integer value) -> 0
        )
        .build();
```

### Changing the type of configuration elements

Changing the type of configuration elements is not supported. If you change the
type of one of these but your configuration file still contains a value of the
old type, a type mismatch will occur when loading a configuration from that
file. Instead, remove the old element and add a new one with a different name.

### Recursive type definitions

Recursive type definitions are currently not allowed but might be supported in a
future version if this feature is requested.

<details>
 <summary>Examples of recursive type definitions</summary>

Neither direct nor indirect recursive type definitions are supported.

```java
public final class RecursiveTypDefinitions {
    // Direct recursive definition
    @Configuration
    static final class R {
        R r;
    }

    // Indirect recursive definition
    @Configuration
    static final class R1 {
        R2 r2;
    }

    @Configuration
    static final class R2 {
        R1 r1;
    }
}
```

</details>

## Project and repository structure

This project contains three classes of modules:

* The `configlib-core` module contains most of the logic of this library. In it,
  you can find (among other things), the object mapper that converts
  configuration instances to maps (and vice versa), most serializers, and the
  classes responsible for the extraction of comments. It does not contain
  anything Minecraft related.
* The `configlib-yaml` module contains the classes that can save configuration
  instances as YAML files and instantiate new instances from such files. This
  module does not contain anything Minecraft related, either.
* The `configlib-paper`, `configlib-velocity`, and `configlib-waterfall` modules
  contain basic plugins that are used to conveniently load this library. These
  three modules shade the `-core` module, the `-yaml` module, and the YAML
  parser when the `shadowJar` task is executed. The shaded jar files are
  released on the [releases page](https://github.com/Exlll/ConfigLib/releases).
    * The `configlib-paper` module additionally contains
      the `ConfigLib.BUKKIT_DEFAULT_PROPERTIES` object which adds support for
      the serialization of Bukkit classes like `ItemStack` as
      described [here](#support-for-bukkit-classes-like-itemstack).

The GitHub repository of this project uses two branches:

* The `master` branch contains the functionality of the latest release version.
* The `dev` branch contains the newest, possibly unstable features and
  refactorings.

**If you plan to contribute to this project, please base your commits on
the `dev` branch.**

## Import

To use this library, import it into your project with Maven or Gradle. Examples
of how to do that are at the end of this section within the spoilers. Currently,
there are three repositories from which you can choose:
- [Maven Central](https://central.sonatype.com/search?q=de.exlll) (recommended),
- [jitpack.io](https://jitpack.io/#Exlll/ConfigLib), and
- GitHub (which [requires authentication](https://github.com/Exlll/ConfigLib/issues/12))

This library has additional dependencies (namely, a YAML parser) which are not
exposed by the artifact you import. You can download _plugin versions_ of this
library that bundle all its dependencies. The artifacts of these versions can be
found on the [releases page](https://github.com/Exlll/ConfigLib/releases) where
you can identify them by their `-paper-`, `-waterfall-`, and `-velocity-` infix
and `-all` suffix. Except for the `-paper-` version, the other plugin versions
currently do not add any additional features. A benefit of these versions is
that they make it easier for you to update this library if you have written
multiple plugins that use it. If you plan to use these versions, don't forget to
add the plugin as a dependency to the `plugin.yml` (for Paper and Waterfall) or
to the dependencies array (for Velocity) of your own plugin.

Alternatively, if you don't want to use an extra plugin, you can shade
the `-yaml` version with its YAML parser yourself.

### Import examples

If you want serialization support for Bukkit classes like `ItemStack`,
replace `configlib-yaml` with `configlib-paper`
(see [here](#support-for-bukkit-classes-like-itemstack)).

#### Import via Maven Central

**Maven**

```xml
<dependency>
    <groupId>de.exlll</groupId>
    <artifactId>configlib-yaml</artifactId>
    <version>4.5.0</version>
</dependency>
```

**Gradle**

```kotlin
repositories { mavenCentral() }

dependencies { implementation("de.exlll:configlib-yaml:4.5.0") }
```

<details>
 <summary>
    Import via <a href="https://jitpack.io/#Exlll/ConfigLib">jitpack.io</a>
 </summary>

**Maven**

```xml 
<repository>
    <id>jitpack.io</id>
    <url>https://jitpack.io</url>
</repository>

<dependency>
    <groupId>com.github.Exlll.ConfigLib</groupId>
    <artifactId>configlib-yaml</artifactId>
    <version>v4.5.0</version>
</dependency>
```

**Gradle**

```groovy
repositories { maven { url 'https://jitpack.io' } }

dependencies { implementation 'com.github.Exlll.ConfigLib:configlib-yaml:v4.5.0' }
```

```kotlin
repositories { maven { url = uri("https://jitpack.io") } }

dependencies { implementation("com.github.Exlll.ConfigLib:configlib-yaml:v4.5.0") }
```

</details>

<details>
 <summary>
    Import via GitHub
 </summary>

Importing via GitHub requires authentication. Check
this [issue](https://github.com/Exlll/ConfigLib/issues/12) if you have any
trouble with that.

**Maven**

```xml 
<repository>
    <id>de.exlll</id>
    <url>https://maven.pkg.github.com/Exlll/ConfigLib</url>
</repository>

<dependency>
    <groupId>de.exlll</groupId>
    <artifactId>configlib-yaml</artifactId>
    <version>4.5.0</version>
</dependency>
```

**Gradle**

```groovy
repositories { maven { url 'https://maven.pkg.github.com/Exlll/ConfigLib' } }

dependencies { implementation 'de.exlll:configlib-yaml:4.5.0' }
```

```kotlin
repositories { maven { url = uri("https://maven.pkg.github.com/Exlll/ConfigLib") } }

dependencies { implementation("de.exlll:configlib-yaml:4.5.0") }
```

</details>

## Future work

This section contains ideas for upcoming features. If you want any of these to
happen any time soon,
please [open an issue](https://github.com/Exlll/ConfigLib/issues/new) where we
can discuss the details.

- JSON, TOML, XML support
- More features and control over updating/versioning
- More control over the ordering of fields, especially in parent/child class
  scenarios
- Recursive definitions
- Shadowing of fields