driver-core/src/main/com/mongodb/internal/connection/RequestMessage.java
protected void backpatchMessageLength(final int startPosition, final BsonOutput bsonOutput) {
        int messageLength = bsonOutput.getPosition() - startPosition;
        bsonOutput.writeInt32(bsonOutput.getPosition() - messageLength, messageLength);
    }
protected void backpatchMessageLength(final int startPosition, final BsonOutput bsonOutput) {
        int messageLength = bsonOutput.getPosition() - startPosition;
        bsonOutput.writeInt32(bsonOutput.getPosition() - messageLength, messageLength);
    }
protected void backpatchMessageLength ( final int startPosition , final BsonOutput bsonOutput ) { int messageLength = bsonOutput . getPosition ( ) - startPosition ; bsonOutput . writeInt32 ( bsonOutput . getPosition ( ) - messageLength , messageLength ) ; }
Backpatches the message length into the beginning of the message.

@param startPosition the start position of the message
@param bsonOutput the output
Backpatches the message length into the beginning of the message .