#Object Logger

What is is?

The Object Logger is a logging framework for logging Objects to a data source. It also contains some nice functions to read from the log file.

##How to use it?

The ObjectLogger is the entry point and requires an IDataSource to write the data to. One ObjectLogger will log all Objects in the same way to one file. Objects are logged with a timestamp, so in actual fact the object that is logged is a TimestampedObject which essentially just wraps the logged Object and contains a timestamp of when it was created.

There is one implementation of IDataSource available for use out of the box and that is the FileSystemDataSource which, you guessed it, writes to the file system. This data source takes a filename and ITimestampedObjectSerialiser to serialise the Object to the file. The default filename is the simple name of the generic type of the ObjectLogger. The default ISerialiser is the TimestampedObjectJsonSerialiser.

The FileSystemDataSource can also take an IRollingStrategy to make decisions on when to roll the log file. By default the DailyRollingStrategy is used, but you can choose to use a MinuteRollingStrategy if you prefer.

The IDataSource also contains methods to read the Objects from the log file. The getAll() method and its various overloaded counterparts allow the caller to read in all or some of the Objects from the file. THe Objects are returned in a TimestampedObjectSet which again provides the caller with more ways to retrieve the data they are looking for.

###Logging Example

Say we have an Account object that we want to log as JSON to the filesystem. This is how we would do it.

public class Account {
    private String name;
    private long id;
    // --- emitted for clarity
}

public class AccountLogger {
    public static void main(String[] args) {
        
        // Name of the file starts with "accounts". Default 
        FileSystemDataSource<Account> dataSource = new FileSystemDataSource<Account>("accounts");		
        
        // This will log files in ~/logs/[yyyy-mm-dd]/...
        dataSource.setFileSystemLoggerPath("~/logs");
        
        // This will log Account objects to a file named ~/logs/[yyyy-mm-dd]/accounts-HH-MM.json as a JSON string.
        ObjectLogger<Account> logger = new ObjectLogger<Account>(dataSource);
        
        Account account = new Account("Jane", 12345);
        logger.log(account);
        Account account = new Account("Gary", 43251);
        logger.log(account);
        Account account = new Account("Gary", 9876);
        logger.log(account);
    }
}

If you open the log file it will look something like:

[
{"logTime":1382942847149,"obj":{"name":"Jane","id":12345}},
{"logTime":1382942847150,"obj":{"name":"Gary","id":43251}},
{"logTime":1382942847151,"obj":{"name":"Gary","id":9876}}
]

If you want to view the json you can use something like http://jsonlint.com to format it before opening it in your favourite text reader.

Reading Example

public class AccountReader {
    public static void main(String[] args) {
        FileSystemDataSource<Account> dataSource = new FileSystemDataSource<Account>("accounts");
        
        // This will read files from ~/logs/[yyyy-mm-dd]/...
        dataSource.setFileSystemLoggerPath("~/logs");		
        TimestampedObjectSet<Account> accounts = dataSource.getAll(type);
        
        Set<TimestampedObject<Account>> uniqueAccounts = accounts.getUniqueForDate(new Date(), new Comparator<TimestampedObject<Account>>{
        
            @Override
            public int compare(TimestampedObject<Account> o1, TimestampedObject<Account> o2) {
                
                // Compare the names of the Accounts within the TimestampedObject
                if (o1.getObj().getName().equals(o2.getObj().getName())) {
                    return 0;
                }
                return 1;
            }
        });
        
        for(TimestampedObject<Account> to : uniqueAccounts) {
            Account account = to.getObj();
            System.out.println(account.getName()+ " : " + account.getId())
        }
    }
}

Should print:

Jane : 12345
Gary : 9876