Java - ObjectOutputStream writeStreamHeader() method

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Description

The Java ObjectOutputStream writeStreamHeader() method is provided so subclasses can append or prepend their own header to the stream. It writes the magic number and version to the stream.

• It's automatically called when an ObjectOutputStream is created.

• It writes standard stream metadata (0xACED0005 by default).

• You can override it in a subclass to −

  • Add custom headers

  • Write version information

  • Integrate with custom protocols

Declaration

Following is the declaration for java.io.ObjectOutputStream.writeStreamHeader() method.

public void writeStreamHeader()

Parameters

• NA

Return Value

This method does not return a value.

Exception

• IOException − If I/O errors occur while writing to the underlying stream.

Example - Add a custom version tag at the beginning of the stream

The following example shows the usage of ObjectOutputStream writeStreamHeader() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.OutputStream;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException {
      try (CustomStream oos = new CustomStream(new FileOutputStream("header1.ser"))) {
         oos.writeObject("Hello with custom header");
      }
   }

   static class CustomStream extends ObjectOutputStream {
      public CustomStream(OutputStream out) throws IOException {
         super(out);
      }

      @Override
      protected void writeStreamHeader() throws IOException {
         super.writeStreamHeader(); // keep default header
         writeUTF("stream-version:1.0"); // add custom tag
         System.out.println("Custom header written: stream-version:1.0");
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Custom header written: stream-version:1.0

Explanation

• Writes default Java stream header.

• Then adds a custom version string.

• Useful for version-aware deserialization later.

Example - Write a completely custom header (skip default)

The following example shows the usage of ObjectOutputStream writeStreamHeader() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.OutputStream;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException {
      try (RawHeaderStream oos = new RawHeaderStream(new FileOutputStream("header2.ser"))) {
         oos.writeObject("Custom-only header");
      }
   }

   static class RawHeaderStream extends ObjectOutputStream {
      public RawHeaderStream(OutputStream out) throws IOException {
         super(out);
      }

      @Override
      protected void writeStreamHeader() throws IOException {
         // Skip default header and write your own 4-byte magic
         writeInt(0xDEADBEEF); // custom marker
         System.out.println("Wrote raw header: 0xDEADBEEF");
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Wrote raw header: 0xDEADBEEF

Explanation

• Replaces the default Java header entirely.

• Useful when interoperating with non-Java readers or enforcing a custom file format.

• Requires custom ObjectInputStream to handle it.

Example - Add a timestamp to the stream header

The following example shows the usage of ObjectOutputStream writeStreamHeader() method.

ObjectOutputStreamDemo.java

package com.tutorialspoint;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.OutputStream;

public class ObjectOutputStreamDemo {
   public static void main(String[] args) throws IOException {
      try (TimestampedStream oos = new TimestampedStream(new FileOutputStream("header3.ser"))) {
         oos.writeObject("Stream with timestamp");
      }
   }

   static class TimestampedStream extends ObjectOutputStream {
      public TimestampedStream(OutputStream out) throws IOException {
         super(out);
      }

      @Override
      protected void writeStreamHeader() throws IOException {
         super.writeStreamHeader(); // retain default
         writeLong(System.currentTimeMillis()); // custom timestamp
         System.out.println("Wrote timestamp to header.");
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Wrote timestamp to header.

Explanation

• Keeps standard header, then writes a timestamp.

• Useful for tracking when a stream was created.

• The corresponding ObjectInputStream must read this timestamp before readObject().