ItemWriter API

The ItemWriter<O> trait defines how to write batches of items to any destination. Writers receive chunks of items for efficient batch operations.

Trait Definition

pub trait ItemWriter<O> {
    /// Writes a batch of items
    fn write(&self, items: &[O]) -> Result<(), BatchError>;

    /// Opens the writer (called once at start)
    fn open(&self) -> Result<(), BatchError> { Ok(()) }

    /// Closes the writer (called once at end)
    fn close(&self) -> Result<(), BatchError> { Ok(()) }

    /// Flushes any buffered data
    fn flush(&self) -> Result<(), BatchError> { Ok(()) }
}

Tip

Writers receive slices of items (chunks) rather than individual items for optimal I/O performance.

Type Alias

pub type ItemWriterResult = Result<(), BatchError>;

Lifecycle Methods

Method	Called When	Purpose
open()	Before first write	Initialize resources (files, connections)
write()	For each chunk	Write batch of items
flush()	After each write	Flush buffers to ensure data persistence
close()	After all writes	Release resources and finalize output

Available Implementations

Spring Batch RS provides 9 built-in writer implementations:

Writer	Feature Flag	Destination	Description
CsvItemWriter<O, W>	csv	CSV files	Writes CSV records with headers
JsonItemWriter<O, W>	json	JSON files	Writes JSON arrays
XmlItemWriter<O, W>	xml	XML files	Writes XML documents
PostgresItemWriter<O>	rdbc-postgres	PostgreSQL	Bulk inserts to PostgreSQL
MysqlItemWriter<O>	rdbc-mysql	MySQL/MariaDB	Bulk inserts to MySQL
SqliteItemWriter<O>	rdbc-sqlite	SQLite	Bulk inserts to SQLite
MongodbItemWriter<O>	mongodb	MongoDB	Bulk inserts to MongoDB
OrmItemWriter<O>	orm	SeaORM	ORM-based database writing
LoggerWriter	built-in	Logs	Debug output via logging

---

CSV Writer

Note

Feature flag: csv

Builder Methods

pub struct CsvItemWriterBuilder<O: Serialize, W: Write> { /* ... */ }

Method	Type	Default	Description
has_headers(bool)	bool	true	Write header row
delimiter(u8)	u8	b','	Field delimiter character
from_path(&str)	-	-	Write to file path
from_writer(W)	-	-	Write to any Write destination

Example

To File

use spring_batch_rs::item::csv::CsvItemWriterBuilder;
use serde::Serialize;

#[derive(Serialize)]
struct Product {
    id: u32,
    name: String,
    price: f64,
}

let writer = CsvItemWriterBuilder::new()
    .has_headers(true)
    .delimiter(b',')
    .from_path("products.csv")?;

To Buffer

use std::io::Cursor;

let buffer = Cursor::new(Vec::new());

let writer = CsvItemWriterBuilder::<Product>::new()
    .has_headers(true)
    .from_writer(buffer);

Custom Delimiter

let writer = CsvItemWriterBuilder::new()
    .has_headers(true)
    .delimiter(b';')  // Semicolon-separated
    .from_path("data.csv")?;

Caution

If the file exists, it will be overwritten. Use append mode if you need to add to existing files.

---

JSON Writer

Note

Feature flag: json

Builder Methods

pub struct JsonItemWriterBuilder<O: Serialize, W: Write> { /* ... */ }

Method	Type	Default	Description
pretty_formatter(bool)	bool	false	Enable pretty-printing
from_path(&str)	-	-	Write to file path
from_writer(W)	-	-	Write to any Write destination

Example

Pretty JSON

use spring_batch_rs::item::json::JsonItemWriterBuilder;
use serde::Serialize;

#[derive(Serialize)]
struct User {
    id: u32,
    name: String,
    email: String,
}

let writer = JsonItemWriterBuilder::<User>::new()
    .pretty_formatter(true)  // Indented, readable output
    .from_path("users.json")?;

Compact JSON

let writer = JsonItemWriterBuilder::<User>::new()
    .pretty_formatter(false)  // Minified output
    .from_path("users.json")?;

Tip

Use pretty_formatter(true) for human-readable output, false for production/API consumption.

---

XML Writer

Note

Feature flag: xml

Builder Methods

pub struct XmlItemWriterBuilder<O: Serialize, W: Write> { /* ... */ }

Method	Type	Default	Description
root_tag(&str)	&str	"root"	Root element name
item_tag(&str)	&str	required	Item element name
from_path(&str)	-	-	Write to file path
from_writer(W)	-	-	Write to any Write destination

Example

use spring_batch_rs::item::xml::XmlItemWriterBuilder;
use serde::Serialize;

#[derive(Serialize)]
#[serde(rename = "vehicle")]
struct Vehicle {
    #[serde(rename = "@type")]
    vehicle_type: String,
    make: String,
    model: String,
    year: i32,
}

let writer = XmlItemWriterBuilder::new()
    .root_tag("vehicles")
    .item_tag("vehicle")
    .from_path("output.xml")?;

Output:

<?xml version="1.0" encoding="UTF-8"?>
<vehicles>
  <vehicle type="car">
    <make>Toyota</make>
    <model>Camry</model>
    <year>2023</year>
  </vehicle>
</vehicles>

Tip

Use #[serde(rename = "@field")] for XML attributes in your structs.

---

PostgreSQL Writer

Note

Feature flag: rdbc-postgres

Builder Methods

pub struct PostgresItemWriterBuilder<O> { /* ... */ }

Method	Type	Description
pool(PgPool)	PgPool	PostgreSQL connection pool
table(&str)	&str	Target table name
binder(Fn)	Fn(&mut QueryBuilder, &O)	Function to bind item fields

Example

use spring_batch_rs::item::rdbc::postgres::PostgresItemWriterBuilder;
use sqlx::PgPool;
use serde::{Deserialize, Serialize};

#[derive(Debug, Clone, Deserialize, Serialize)]
struct Person {
    first_name: String,
    last_name: String,
    email: String,
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let pool = PgPool::connect("postgres://user:pass@localhost/db").await?;

    let writer = PostgresItemWriterBuilder::new()
        .pool(pool)
        .table("persons")
        .binder(|query, person: &Person| {
            query
                .push_values([person], |mut b, p| {
                    b.push_bind(&p.first_name)
                     .push_bind(&p.last_name)
                     .push_bind(&p.email);
                });
        })
        .build();

    Ok(())
}

Note

The writer uses PostgreSQL’s bulk insert capabilities for optimal performance.

---

MySQL Writer

Note

Feature flag: rdbc-mysql

Builder Methods

Same as PostgreSQL writer, but use MySqlPool:

use spring_batch_rs::item::rdbc::mysql::MysqlItemWriterBuilder;
use sqlx::MySqlPool;

let pool = MySqlPool::connect("mysql://user:pass@localhost/db").await?;

let writer = MysqlItemWriterBuilder::new()
    .pool(pool)
    .table("persons")
    .binder(|query, person: &Person| {
        query.push_values([person], |mut b, p| {
            b.push_bind(&p.first_name)
             .push_bind(&p.last_name)
             .push_bind(&p.email);
        });
    })
    .build();

---

SQLite Writer

Note

Feature flag: rdbc-sqlite

Builder Methods

Same as PostgreSQL writer, but use SqlitePool:

use spring_batch_rs::item::rdbc::sqlite::SqliteItemWriterBuilder;
use sqlx::SqlitePool;

let pool = SqlitePool::connect("sqlite::memory:").await?;

let writer = SqliteItemWriterBuilder::new()
    .pool(pool)
    .table("persons")
    .binder(|query, person: &Person| {
        query.push_values([person], |mut b, p| {
            b.push_bind(&p.first_name)
             .push_bind(&p.last_name)
             .push_bind(&p.email);
        });
    })
    .build();

---

MongoDB Writer

Note

Feature flag: mongodb

Builder Methods

pub struct MongodbItemWriterBuilder<O> { /* ... */ }

Method	Type	Description
collection(&Collection<O>)	&Collection<O>	MongoDB collection

Example

use spring_batch_rs::item::mongodb::MongodbItemWriterBuilder;
use mongodb::{sync::Client, bson::doc};
use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize, Serialize, Clone)]
struct Book {
    title: String,
    author: String,
    isbn: String,
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = Client::with_uri_str("mongodb://localhost:27017")?;
    let db = client.database("library");
    let collection = db.collection::<Book>("books");

    let writer = MongodbItemWriterBuilder::new()
        .collection(&collection)
        .build();

    Ok(())
}

Tip

The writer uses MongoDB’s insert_many() for efficient bulk inserts.

---

ORM Writer

Note

Feature flag: orm

Writes entities using SeaORM. Your entity must implement SeaORM’s ActiveModelTrait.

Example

use spring_batch_rs::item::orm::OrmItemWriterBuilder;
use sea_orm::{Database, DatabaseConnection};

// Assuming you have a SeaORM entity
use entity::person::{Entity as PersonEntity, ActiveModel};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let db: DatabaseConnection = Database::connect("sqlite::memory:").await?;

    let writer = OrmItemWriterBuilder::new()
        .entity(PersonEntity)
        .connection(&db)
        .build();

    Ok(())
}

---

Logger Writer

Note

Built-in - No feature flag required

Writes items to logs for debugging purposes.

Builder Methods

pub struct LoggerItemWriterBuilder { /* ... */ }

Method	Type	Default	Description
log_level(Level)	log::Level	Info	Logging level

Example

use spring_batch_rs::item::logger::LoggerItemWriterBuilder;
use log::Level;

// Log at INFO level
let writer = LoggerItemWriterBuilder::new()
    .log_level(Level::Info)
    .build();

// Log at DEBUG level
let writer = LoggerItemWriterBuilder::new()
    .log_level(Level::Debug)
    .build();

Tip

Perfect for development and debugging without writing to actual files.

---

Custom Implementation

You can implement ItemWriter for any destination:

use spring_batch_rs::core::item::{ItemWriter, ItemWriterResult};
use spring_batch_rs::error::BatchError;
use std::sync::Mutex;

struct MyCustomWriter {
    data: Mutex<Vec<String>>,
}

impl ItemWriter<String> for MyCustomWriter {
    fn write(&self, items: &[String]) -> ItemWriterResult {
        let mut data = self.data.lock().unwrap();
        data.extend_from_slice(items);
        Ok(())
    }

    fn open(&self) -> ItemWriterResult {
        println!("Opening writer");
        Ok(())
    }

    fn close(&self) -> ItemWriterResult {
        println!("Closing writer, wrote {} items", self.data.lock().unwrap().len());
        Ok(())
    }

    fn flush(&self) -> ItemWriterResult {
        println!("Flushing writer");
        Ok(())
    }
}

impl MyCustomWriter {
    fn new() -> Self {
        Self {
            data: Mutex::new(Vec::new()),
        }
    }
}

Caution

Thread Safety: Writers must be Send + Sync. Use Mutex, RwLock, or atomic types for internal state.

Transaction Handling

Writers can implement transactional behavior:

use std::sync::Mutex;

struct TransactionalWriter {
    buffer: Mutex<Vec<String>>,
    committed: Mutex<Vec<String>>,
}

impl ItemWriter<String> for TransactionalWriter {
    fn write(&self, items: &[String]) -> ItemWriterResult {
        // Write to buffer (transaction)
        let mut buffer = self.buffer.lock().unwrap();
        buffer.extend_from_slice(items);
        Ok(())
    }

    fn flush(&self) -> ItemWriterResult {
        // Commit transaction
        let mut buffer = self.buffer.lock().unwrap();
        let mut committed = self.committed.lock().unwrap();
        committed.append(&mut buffer);
        Ok(())
    }

    fn close(&self) -> ItemWriterResult {
        // Final flush on close
        self.flush()
    }
}

Best Practices

Batch Operations

Use bulk insert/write APIs when available for better performance

Buffer Management

Implement flush() to ensure data is persisted, especially for buffered writers

Resource Cleanup

Always release resources in close() even if errors occurred

Error Context

Return descriptive BatchError::ItemWriter errors with details about what failed

Performance Tips

File Writers

• Use buffered I/O (BufWriter) for file-based writers
• Consider chunk size when writing large files
• Use appropriate buffer sizes (8KB-64KB typical)

Database Writers

• Use bulk insert statements instead of individual inserts
• Enable connection pooling
• Consider batch size vs transaction overhead
• Use prepared statements when possible

Network Writers

• Implement retry logic for transient failures
• Use connection pooling
• Consider timeout settings
• Implement proper error handling for network issues

See Also

• ItemReader API - Reading data sources
• ItemProcessor API - Transforming data
• Database Examples - Database writer examples
• JSON Examples - JSON writer examples