What are thread local variables?

The more accurate name of ThreadLocal translated into Chinese should be: thread local variable.

What is the use of this thing, or why is there such a thing? Let me explain first. During concurrent programming, member variables are actually thread-unsafe if no processing is done. It is obviously not possible for each thread to operate the same variable, and we also know that the keyword volatile does not guarantee thread security. safe. So in one case, we need to meet such a condition: the variable is the same, but each thread uses the same initial value, that is, uses a new copy of the same variable. In this case, ThreadLocal is very useful. For example, DAO's database connection. We know that DAO is a singleton, so its attribute Connection is not a thread-safe variable. And each of our threads needs to use it, and each uses its own. In this case, ThreadLocal solves this problem better.

We analyze this problem from the perspective of source code.

First define a ThreadLocal:

   ThreadLocal<connection> tl =  ThreadLocal<connection>  Connection initConn = = DriverManager.getConnection("url, name and password"=( ==</connection></connection>

package java.lang;import java.lang.ref.*;import java.util.concurrent.atomic.AtomicInteger;public class ThreadLocal<t> {   private final int threadLocalHashCode = nextHashCode();   private static AtomicInteger nextHashCode =new AtomicInteger();private static final int HASH_INCREMENT = 0x61c88647;private static int nextHashCode() {return nextHashCode.getAndAdd(HASH_INCREMENT);
    }protected T initialValue() {return null;
    }public ThreadLocal() {
    }public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);if (e != null)return (T)e.value;
        }return setInitialValue();
    }private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);if (map != null)
            map.set(this, value);elsecreateMap(t, value);return value;
    }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;
    }void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {return new ThreadLocalMap(parentMap);
    }T childValue(T parentValue) {throw new UnsupportedOperationException();
    }static class ThreadLocalMap {static class Entry extends WeakReference<threadlocal> {/** The value associated with this ThreadLocal. */Object value;

            Entry(ThreadLocal k, Object v) {super(k);
                value = v;
            }
        }private static final int INITIAL_CAPACITY = 16;private Entry[] table;private int size = 0;private int threshold; // Default to 0private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }private static int nextIndex(int i, int len) {return ((i + 1 = 0) ? i - 1 : len - 1);
        }ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];for (int j = 0; j = threshold)
                rehash();
        }private void remove(ThreadLocal key) {
            Entry[] tab = table;int len = tab.length;int i = key.threadLocalHashCode & (len-1);for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {if (e.get() == key) {
                    e.clear();
                    expungeStaleEntry(i);return;
                }
            }
        }private void replaceStaleEntry(ThreadLocal key, Object value,                                       int staleSlot) {
            Entry[] tab = table;int len = tab.length;
            Entry e;int slotToExpunge = staleSlot;for (int i = prevIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = prevIndex(i, len))if (e.get() == null)
                    slotToExpunge = i;for (int i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal k = e.get();if (k == key) {
                    e.value = value;

                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);return;
                }if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }private int expungeStaleEntry(int staleSlot) {
            Entry[] tab = table;int len = tab.length;tab[staleSlot].value = null;
            tab[staleSlot] = null;
            size--;            Entry e;int i;for (i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal k = e.get();if (k == null) {
                    e.value = null;
                    tab[i] = null;
                    size--;
                } else {int h = k.threadLocalHashCode & (len - 1);if (h != i) {
                        tab[i] = null;while (tab[h] != null)
                            h = nextIndex(h, len);
                        tab[h] = e;
                    }
                }
            }return i;
        }private boolean cleanSomeSlots(int i, int n) {boolean removed = false;
            Entry[] tab = table;int len = tab.length;do {
                i = nextIndex(i, len);
                Entry e = tab[i];if (e != null && e.get() == null) {
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);return removed;
        }private void rehash() {
            expungeStaleEntries();// Use lower threshold for doubling to avoid hysteresisif (size >= threshold - threshold / 4)
                resize();
        }private void resize() {
            Entry[] oldTab = table;int oldLen = oldTab.length;int newLen = oldLen * 2;
            Entry[] newTab = new Entry[newLen];int count = 0;for (int j = 0; j </threadlocal></t>

In this way, the same connection is used, but each The connections are all new and are copies of the same connection.

So what is the implementation mechanism?

1. Each Thread object maintains a ThreadLocalMap internally. Such a ThreadLocal Map can store several ThreadLocals

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

2. When we call get( ) method, first get the current thread, and then get the ThreadLocalMap object of the current thread. If it is not empty, then take out the value of ThreadLocal, otherwise initialize it. The initialization is to set the value of initialValue into ThreadLocal.

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);if (e != null)return (T)e.value;
    }return setInitialValue();
}

3. When we call the set() method, it is very common to set the value into ThreadLocal.

4. Summary: When we call the get method, there is actually a ThreadLocal in each current thread. Each acquisition or setting is an operation on the ThreadLocal, which is separate from other threads.

5. Application scenario: ThreadLocal is most suitable when many threads need to use the same object multiple times and the object needs to have the same initialization value.

6. In fact, no matter how much I say, it is better to look at the source code to make it clearer. If you want to look at the source code, it involves a WeakReference and a Map. You need to understand these two places. These two things are weak references of a.Java, that is, the references wrapped (referenced) will be destroyed during GC. Object, this threadLocal as a key may be destroyed, but as long as the class we define is not unloaded, the strong reference of tl will always refer to this ThreadLocal and will never be deleted by gc. b. Similar to HashMap.

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