Find out what's going on in Velocity, the interaction during rendering between a template and the context passed in. Bad news though, side-effects can happen which really forbids caching the variables locally.

TestContext.java

import java.io.OutputStreamWriter;
import java.io.Writer;
import java.util.Arrays;

import org.apache.velocity.VelocityContext;
import org.apache.velocity.app.Velocity;
import org.apache.velocity.context.Context;

/**
 * Demonstrate interaction between a template and its context.
 * Side-effects can arise from such interaction, which forbids
 * a lot of optimizations if this behavior has to be kept.
 * 
 * @author RednaxelaFX
 */
public class TestContext {

	/**
	 * @param args
	 */
	public static void main(String[] args) throws Exception {
		String template = "#set ($a = 123)#foreach ($i in [8..9])#end";
		Context context = new VelocityContext() {
			@Override
			public Object internalGet(String key) {
				Object value = super.internalGet(key);
				System.out.println("read from " + key + " = " + value);
				return value;
			}

			@Override
			public Object internalPut(String key, Object value) {
				System.out.println("written to " + key + " = " + value);
				return super.internalPut(key, value);
			}

			@Override
			public boolean internalContainsKey(Object key) {
				boolean containsKey = super.internalContainsKey(key);
				System.out.println("contains key " + key + " = " + containsKey);
				return containsKey;
			}

			@Override
			public Object[] internalGetKeys() {
				Object[] keys = super.internalGetKeys();
				System.out.println("got keys " + Arrays.toString(keys));
				return keys;
			}

			@Override
			public Object internalRemove(Object key) {
				System.out.println("remove " + key);
				return super.internalRemove(key);
			}
		};
		Writer writer = new OutputStreamWriter(System.out);
		Velocity.evaluate(context, writer, "error", template);
		writer.close();
	}
}

/*
Output:
written to a = 123
read from i = null
read from velocityCount = null
read from velocityHasNext = null
written to velocityCount = 1
written to velocityHasNext = true
written to i = 8
written to velocityCount = 2
written to velocityHasNext = false
written to i = 9
remove velocityCount
remove i
remove velocityHasNext
 */