currentdatabase.java

Java的面向对象数据库系统的源代码

/* CurrentDatabase.java
*/

package org.ozoneDB;

import org.ozoneDB.data.SimpleArrayList;

/**
    A method for maintaining an implicit reference to the current database
    
    @author <A HREF="http://www.medium.net/">Xu鈔 Baldauf,Medium.net</A>
*/
public class CurrentDatabase {
    
    protected static ThreadLocal instances = new ThreadLocal() {
        protected Object initialValue() {
            return new CurrentDatabase();
        }
    };
    
    protected SimpleArrayList stack;
    
    protected CurrentDatabase() {
        stack = new SimpleArrayList(6);
    }
    
    protected void addCurrentDatabase(OzoneInterface newCurrentDatabase) {
        stack.push(newCurrentDatabase);
    }
    
    protected void removeCurrentDatabase() {
        Object result = stack.pop();
        
        assert result!=null;
    }
    
    protected OzoneInterface getCurrentDatabase() {
        OzoneInterface currentDatabase = (OzoneInterface) stack.peek();
        
        // Maybe we should asserts instead?
        if (currentDatabase==null) {
            throw new IllegalStateException("There is no current database defined for this thread "+Thread.currentThread()+".");
        }
        
        return currentDatabase;
    }
    
    /**
        Registers a new current database.
        Every call to this method must be matched by a call to {@link #unregister}.
        Thus, the usage should be as following:
        
        <PRE>
          CurrentDatabase.register(database);
            
            try {
                ... do something useful ...
            } finally {
                CurrentDatabase.unregister();
            }
        </PRE>
        
        <DIV>
            Registering current databases may be nested. If already a current database is registered,
            registering a new one hides the old one until {@link #unregister} is called.
        </DIV>

        <DIV>       
            Registered current databases only apply to the current thread. Each thread has to register
            its current database separately.
        </DIV>
    */
    public static void register(OzoneInterface newCurrentDatabase) {
        assert newCurrentDatabase!=null;
        
        ((CurrentDatabase) instances.get()).addCurrentDatabase(newCurrentDatabase);
    }

    /**
        Unregisters the current database. The database which was current before the matching call to
        {@link #register} will be the new current database.  
    */  
    public static void unregister() {
        ((CurrentDatabase) instances.get()).removeCurrentDatabase();
    }
    
    /**
        Returns a reference to the current database.
        
        @return
            a reference to the current database
        @throws IllegalStateException
            If no current database is set.
    */
    public static OzoneInterface get() throws IllegalStateException {
        return ((CurrentDatabase) instances.get()).getCurrentDatabase();
    }
}

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