Migrate OkHttp to bramble-core

bramble-api/src/main/java/org/briarproject/bramble/api/WeakSingletonProvider.java

@@ -0,0 +1,35 @@
+package org.briarproject.bramble.api;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.lang.ref.WeakReference;
+
+import javax.annotation.concurrent.GuardedBy;
+import javax.inject.Provider;
+
+/**
+ * A {@link Provider} that keeps a {@link WeakReference} to the last provided
+ * instance and provides the same instance again until the instance is garbage
+ * collected.
+ */
+@NotNullByDefault
+public abstract class WeakSingletonProvider<T> implements Provider<T> {
+
+	private final Object lock = new Object();
+	@GuardedBy("lock")
+	private WeakReference<T> ref = new WeakReference<>(null);
+
+	@Override
+	public T get() {
+		synchronized (lock) {
+			T instance = ref.get();
+			if (instance == null) {
+				instance = createInstance();
+				ref = new WeakReference<>(instance);
+			}
+			return instance;
+		}
+	}
+
+	public abstract T createInstance();
+}