Caching Reloading Put Block - multithreading

Question

how can i block the put method when cache reloading is being called.
Example: These are dummy classes not the actual.
Caching class
public class Class1 {
private static Map<Integer, Integer> map = new HashMap<>();
public Class1() {
for (int i = 0; i < 20; i++) {
map.put(i, ++i);
}
}
public void reload() throws InterruptedException {
Map<Integer, Integer> exist = map;
System.out.println("are you waiting ");
System.out.println("waiting over");
map = new HashMap<>();
for (Map.Entry<Integer, Integer> entry : exist.entrySet()) {
map.put(entry.getKey(), entry.getValue());
}
for (int i = 100; i < 120; i++) {
map.put(i, ++i);
}
}
public Map<Integer, Integer> getMap() {
return map;
}
}
Class initializing the cache
public class Class2 {
private static Class1 cache = new Class1();;
public Class1 getCache() {
return cache;
}
public void reload() throws InterruptedException {
cache.reload();
}
}
class using the cache
package com.diaryreaders.corejava.algorithms.dp;
public class Class3 {
public static void main(String[] args) {
final Class2 klass = new Class2();
Runnable runn1 = new Runnable() {
#Override
public void run() {
int i = 50, j = 50;
while (i < 100) {
System.out.println("I am stuck due to lock");
klass.getCache().getMap().put(i++, j++);
System.out.println(klass.getCache().getMap());
}
}
};
Runnable runn2 = new Runnable() {
#Override
public void run() {
try {
System.out.println("Calling reloading");
klass.reload();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
Thread t1 = new Thread(runn1);
Thread t2 = new Thread(runn2);
t1.start();
t2.start();
}
}
As thread t2 is calling reloading, t1 should be blocked i.e cache put method be blocked till reloading completes