WriteOnceProperties.java

package edu.umd.cs.findbugs.util;

import java.util.HashMap;
import java.util.Map;
import java.util.Properties;


public class WriteOnceProperties  extends Properties {

	private static final long serialVersionUID = 1L;
	static class PropertyReadAt extends Exception {
		private static final long serialVersionUID = 1L;};
	private Map<String, PropertyReadAt> propertReadAt = new HashMap<String, PropertyReadAt>();
	
	private WriteOnceProperties(Properties initialValue) {
		super.putAll(initialValue);
	}
	
	@Override
	public boolean equals(Object o) {
		return super.equals(o);
	}
	
	@Override
	public int hashCode() {
		return super.hashCode();
	}
	@Override
	public String getProperty(String key) {
		String result =  super.getProperty(key);
		if (result != null && result.length() > 0 && !propertReadAt.containsKey(key))
			propertReadAt.put(key, new PropertyReadAt());
		return result;
	}
	@Override
	public String getProperty(String key, String defaultValue) {
		String result =  super.getProperty(key, defaultValue);
		if (result != null && result.length() > 0 && !propertReadAt.containsKey(key))
			propertReadAt.put(key, new PropertyReadAt());
		return result;
	}
	@Override
	public Object setProperty(String key, String value) {
		if (propertReadAt.containsKey(key) && !value.equals(super.getProperty(key))) {
			IllegalStateException e = new IllegalStateException("Changing property '" + key
					+ "' to '" + value + "' after it has already been read as '" + super.getProperty(key) +"'");
			e.initCause(propertReadAt.get(key));

			throw e;
		}
		return super.setProperty(key, value);
	}
	
	public static void makeSystemPropertiesWriteOnce() {
		Properties properties = System.getProperties();
		if (properties instanceof WriteOnceProperties) return;
		System.setProperties(new WriteOnceProperties(properties));
	}
	public static void main(String args[]) {
		dumpProperties();
		System.out.println("-----");
		makeSystemPropertiesWriteOnce();
		dumpProperties();
		System.setProperty("x","1");
		System.setProperty("y","1");
		System.getProperty("y");
		System.setProperty("x","2");
		System.setProperty("y","2");
		
		
	}

	/**
	 * 
	 */
	private static void dumpProperties() {
		
		Properties properties = System.getProperties();
		System.out.println("Total properties: " + properties.size());
		for(Object k  : properties.keySet()) {
			System.out.println(k + " : " + System.getProperty((String)k));
		}
	}
}