### Define Custom Instance Representations

Source: https://pypi.org/project/javaobj-py3

Define how object data is loaded by inheriting from javaobj.v2.beans.JavaInstance and implementing the load_from_instance method.

```python
class CustomWriterInstance(javaobj.v2.beans.JavaInstance):
    def __init__(self):
        javaobj.v2.beans.JavaInstance.__init__(self)

    def load_from_instance(self):
        """
        Updates the content of this instance
        from its parsed fields and annotations
        :return: True on success, False on error
        """
        if self.classdesc and self.classdesc in self.annotations:
            # Here, we known there is something written before the fields,
            # even if it's not declared in the class description
            fields = ["int_not_in_fields"] + self.classdesc.fields_names
            raw_data = self.annotations[self.classdesc]
            int_not_in_fields = struct.unpack(
                ">i", BytesIO(raw_data[0].data).read(4)
            )[0]
            custom_obj = raw_data[1]
            values = [int_not_in_fields, custom_obj]
            self.field_data = dict(zip(fields, values))
            return True

        return False


class RandomChildInstance(javaobj.v2.beans.JavaInstance):
    def load_from_instance(self):
        """
        Updates the content of this instance
        from its parsed fields and annotations
        :return: True on success, False on error
        """
        if self.classdesc and self.classdesc in self.field_data:
            fields = self.classdesc.fields_names
            values = [
                self.field_data[self.classdesc][self.classdesc.fields[i]]
                for i in range(len(fields))
            ]
            self.field_data = dict(zip(fields, values))
            if (
                self.classdesc.super_class
                and self.classdesc.super_class in self.annotations
            ):
                super_class = self.annotations[self.classdesc.super_class][0]
                self.annotations = dict(
                    zip(super_class.fields_names, super_class.field_data)
                )
            return True

        return False
```

--------------------------------

### Execute Object Loading with Transformers

Source: https://pypi.org/project/javaobj-py3

Pass the list of custom transformers to the javaobj.loads function to process the serialized object.

```python
# Load the object using those transformers
transformers = [
    CustomWriterTransformer(),
    RandomChildTransformer(),
    JavaRandomTransformer()
]
pobj = javaobj.loads("custom_objects.ser", *transformers)
```

--------------------------------

### Serialize Custom Objects to a File

Source: https://pypi.org/project/javaobj-py3

Write custom objects to a file using `ObjectOutputStream`. Ensure to flush and close the stream after writing.

```java
ObjectOutputStream oos = new ObjectOutputStream(
    new FileOutputStream("custom_objects.ser"));
CustomClass writer = new CustomClass();
writer.start(oos);
oos.flush();
oos.close();

```

--------------------------------

### Unmarshall Java Object (V2)

Source: https://pypi.org/project/javaobj-py3

Use the javaobj.v2.load method to parse binary data from a file descriptor.

```python
import javaobj.v2 as javaobj

with open("obj5.ser", "rb") as fd:
    pobj = javaobj.load(fd)

print(pobj.dump())
```

--------------------------------

### Implement Custom Object Transformer (V2)

Source: https://pypi.org/project/javaobj-py3

Define a custom JavaInstance and ObjectTransformer to handle specific Java classes like HashMap during deserialization.

```python
class JavaMap(dict, javaobj.v2.beans.JavaInstance):
    """
    Inherits from dict for Python usage, JavaInstance for parsing purpose
    """
    def __init__(self):
        # Don't forget to call both constructors
        dict.__init__(self)
        JavaInstance.__init__(self)

    def load_from_blockdata(self, parser, reader, indent=0):
    """
    Reads content stored in a block data.

    This method is called only if the class description has both the
    `SC_EXTERNALIZABLE` and `SC_BLOCK_DATA` flags set.

    The stream parsing will stop and fail if this method returns False.

    :param parser: The JavaStreamParser in use
    :param reader: The underlying data stream reader
    :param indent: Indentation to use in logs
    :return: True on success, False on error
    """
    # This kind of class is not supposed to have the SC_BLOCK_DATA flag set
    return False

    def load_from_instance(self, indent=0):
        # type: (int) -> bool
        """
        Load content from the parsed instance object.

        This method is called after the block data (if any), the fields and
        the annotations have been loaded.

        :param indent: Indentation to use while logging
        :return: True on success (currently ignored)
        """
        # Maps have their content in their annotations
        for cd, annotations in self.annotations.items():
            # Annotations are associated to their definition class
            if cd.name == "java.util.HashMap":
                # We are in the annotation created by the handled class
                # Group annotation elements 2 by 2
                # (storage is: key, value, key, value, ...)
                args = [iter(annotations[1:])] * 2
                for key, value in zip(*args):
                    self[key] = value

                # Job done
                return True

        # Couldn't load the data
        return False

class MapObjectTransformer(javaobj.v2.api.ObjectTransformer):
    """
    Creates a JavaInstance object with custom loading methods for the
    classes it can handle
    """
    def create_instance(self, classdesc):
        # type: (JavaClassDesc) -> Optional[JavaInstance]
        """
        Transforms a parsed Java object into a Python object

        :param classdesc: The description of a Java class
        :return: The Python form of the object, or the original JavaObject
        """
        if classdesc.name == "java.util.HashMap":
            # We can handle this class description
            return JavaMap()
        else:
            # Return None if the class is not handled
            return None
```

--------------------------------

### Define Custom Serializable Classes

Source: https://pypi.org/project/javaobj-py3

Define custom classes that implement `Serializable` to control their serialization. The `writeObject` method allows custom logic for serializing object fields.

```java
class CustomClass implements Serializable {

    private static final long serialVersionUID = 1;

    public void start(ObjectOutputStream out) throws Exception {
        this.writeObject(out);
    }

    private void writeObject(ObjectOutputStream out) throws IOException {
        CustomWriter custom = new CustomWriter(42);
        out.writeObject(custom);
        out.flush();
    }
}

class RandomChild extends Random {

    private static final long serialVersionUID = 1;
    private int num = 1;
    private double doub = 4.5;

    RandomChild(int seed) {
        super(seed);
    }
}

class CustomWriter implements Serializable {
    protected RandomChild custom_obj;

    CustomWriter(int seed) {
        custom_obj = new RandomChild(seed);
    }

    private static final long serialVersionUID = 1;
    private static final int CURRENT_SERIAL_VERSION = 0;

    private void writeObject(ObjectOutputStream out) throws IOException {
        out.writeInt(CURRENT_SERIAL_VERSION);
        out.writeObject(custom_obj);
    }
}
```

--------------------------------

### Unmarshall Java Object (V1)

Source: https://pypi.org/project/javaobj-py3

Use the javaobj.loads function to deserialize a Java serialized file.

```python
import javaobj

with open("obj5.ser", "rb") as fd:
    jobj = fd.read()

pobj = javaobj.loads(jobj)
print(pobj)
```

--------------------------------

### Define Custom Object Transformers

Source: https://pypi.org/project/javaobj-py3

Implement custom transformers by inheriting from javaobj.v2.transformers.ObjectTransformer to handle specific Java classes during deserialization.

```python
class BaseTransformer(javaobj.v2.transformers.ObjectTransformer):
    """
    Creates a JavaInstance object with custom loading methods for the
    classes it can handle
    """

    def __init__(self, handled_classes=None):
        self.instance = None
        self.handled_classes = handled_classes or {}

    def create_instance(self, classdesc):
        """
        Transforms a parsed Java object into a Python object

        :param classdesc: The description of a Java class
        :return: The Python form of the object, or the original JavaObject
        """
        if classdesc.name in self.handled_classes:
            self.instance = self.handled_classes[classdesc.name]()
            return self.instance

        return None

class RandomChildTransformer(BaseTransformer):
    def __init__(self):
        super(RandomChildTransformer, self).__init__(
            {"RandomChild": RandomChildInstance}
        )

class CustomWriterTransformer(BaseTransformer):
    def __init__(self):
        super(CustomWriterTransformer, self).__init__(
            {"CustomWriter": CustomWriterInstance}
        )

class JavaRandomTransformer(BaseTransformer):
    def __init__(self):
        super(JavaRandomTransformer, self).__init__()
        self.name = "java.util.Random"
        self.field_names = ["haveNextNextGaussian", "nextNextGaussian", "seed"]
        self.field_types = [
            javaobj.v2.beans.FieldType.BOOLEAN,
            javaobj.v2.beans.FieldType.DOUBLE,
            javaobj.v2.beans.FieldType.LONG,
        ]

    def load_custom_writeObject(self, parser, reader, name):
        if name != self.name:
            return None

        fields = []
        values = []
        for f_name, f_type in zip(self.field_names, self.field_types):
            values.append(parser._read_field_value(f_type))
            fields.append(javaobj.beans.JavaField(f_type, f_name))

        class_desc = javaobj.beans.JavaClassDesc(
            javaobj.beans.ClassDescType.NORMALCLASS
        )
        class_desc.name = self.name
        class_desc.desc_flags = javaobj.beans.ClassDataType.EXTERNAL_CONTENTS
        class_desc.fields = fields
        class_desc.field_data = values
        return class_desc
```

--------------------------------

### Unmarshall Java Object via JavaObjectUnmarshaller (V1)

Source: https://pypi.org/project/javaobj-py3

Use the JavaObjectUnmarshaller class for direct stream processing of serialized objects.

```python
import javaobj

with open("objCollections.ser", "rb") as fd:
    marshaller = javaobj.JavaObjectUnmarshaller(fd)
    pobj = marshaller.readObject()

    print(pobj.value, "should be", 17)
    print(pobj.next, "should be", True)

    pobj = marshaller.readObject()
```

--------------------------------

### Accessing Non-Serialized Field

Source: https://pypi.org/project/javaobj-py3

This snippet demonstrates how to access a static field that is not serialized by default. It is useful for retrieving data that is part of the class but not included in the standard serialization process.

```python
print(pobj.field_data["int_not_in_fields"])
```

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