public class ThreadUnsafe {private static Connection connection = DriverManager.getConnection(DB_URL);public void Connection getConnection{  /* 在多线程应用中，connection 在被多个线程访问 */return connection;}
}

public class ThreadUnsafe {private static ThreadLocal<Connection> connectionHodler = new ThreadLocal<>{public Connection initialValue() {return DriverManager.getConnection(DB_URL);}} public void Connection getConnection{ /* 即使多个线程可以访问，依然安全 */return connectionHolder.get();}
}

 /* ThreadLocal values pertaining to this thread. This map is maintained* by the ThreadLocal class. */ThreadLocal.ThreadLocalMap threadLocals = null;

    /*** 返回ThreadLocal封装的对象。*/public T get() {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null) { /* 首次调用map为null */ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}return setInitialValue(); /* 首次调用的返回值 */}/*** 初始化封装在ThreadLocal中对象的值。*/private T setInitialValue() {T value = initialValue();Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value); //为什么键值是ThreadLocal对象？，因为一个线程对象可能有使用多个ThreadLocal封闭的变量elsecreateMap(t, value);return value;}/*** 更新封装在ThreadLocal中对象的值*/public void set(T value) {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);}
public void remove() {ThreadLocalMap m = getMap(Thread.currentThread());if (m != null)m.remove(this);}ThreadLocalMap getMap(Thread t) {return t.threadLocals;}/*** 创建一个Map，用于保存ThreadLocal和其封装的对象。*/void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue);}