Java程序员必看！ThreadLocal终极指南，你知道它能为你做什么吗?

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1. 引言

• 简介

在当今软件开发领域，多线程编程变得日益普遍。然而，随着多线程的广泛应用，一些经典的问题也浮出水面，其中最显著的之一是线程安全。ThreadLocal作为一个解决多线程环境下共享资源问题的利器，逐渐成为程序员工具箱中的一员。本文将深入探讨ThreadLocal的定义、作用以及在多线程环境中的应用。

2. 多线程基础知识回顾

2.1 多线程概述

多线程是一种并发执行的编程方式，允许程序同时执行多个线程。

2.2 多线程的挑战

• 共享资源的问题

多线程同时访问共享资源可能导致数据不一致性。

• 竞态条件

线程执行顺序不确定可能导致程序出现难以预料的问题。

• 同步和锁的概念

引入同步机制和锁来解决共享资源问题，但可能导致性能问题和复杂性增加。

3. ThreadLocal的概述

3.1ThreadLocal是什么

• 概念解释

ThreadLocal是一个提供线程局部变量的类，每个线程都有一个独立的变量副本。

• 用途和优势

ThreadLocal解决了多线程环境下共享变量的问题，提高了程序的性能和可维护性。

3.2ThreadLocal的实现原理

• 数据结构以及源码

ThreadLocal使用ThreadLocalMap来存储每个线程的变量副本。

package java.lang;
import jdk.internal.misc.TerminatingThreadLocal;

import java.lang.ref.*;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;


public class ThreadLocal {
    //每个ThreadLocal都有一个唯一的标识，用于在ThreadLocalMap中定位值
    private final int threadLocalHashCode = nextHashCode();

    // 下一个可用的哈希码，由AtomicInteger维护
    private static AtomicInteger nextHashCode =
            new AtomicInteger();
    // 哈希增量，用于计算下一个哈希码
    private static final int HASH_INCREMENT = 0x61c88647;

    // 返回下一个可用的哈希码
    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

    // 初始化ThreadLocal的值
    protected T initialValue() {
        return null;
    }
    
    public static  ThreadLocal withInitial(Supplier supplier) {
        return new SuppliedThreadLocal<>(supplier);
    }
    
    public ThreadLocal() {
    }

    // 获取当前线程的ThreadLocal值
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }
    
    boolean isPresent() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        return map != null && map.getEntry(this) != null;
    }

    // 如果当前线程没有值，则设置初始值
    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            map.set(this, value);
        } else {
            createMap(t, value);
        }
        if (this instanceof TerminatingThreadLocal) {
            TerminatingThreadLocal.register((TerminatingThreadLocal) this);
        }
        return value;
    }

    // 设置当前线程的ThreadLocal值
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            map.set(this, value);
        } else {
            createMap(t, value);
        }
    }
    
    // 移除当前线程的ThreadLocalMap中维护的与当前ThreadLocal对象相关的Entry记录
    public void remove() {
        ThreadLocalMap m = getMap(Thread.currentThread());
        if (m != null) {
            m.remove(this);
        }
    }

    // 获取当前线程的ThreadLocalMap
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

    // 设置当前线程的ThreadLocalMap
    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();
    }

    /**
     * An extension of ThreadLocal that obtains its initial value from
     * the specified {@code Supplier}.
     */
    static final class SuppliedThreadLocal extends ThreadLocal {

        private final Supplier supplier;

        SuppliedThreadLocal(Supplier supplier) {
            this.supplier = Objects.requireNonNull(supplier);
        }

        @Override
        protected T initialValue() {
            return supplier.get();
        }
    }

    // ThreadLocalMap 用于存储ThreadLocal和其对应的值
    static class ThreadLocalMap {

        static class Entry extends WeakReference> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }

        //The initial capacity -- MUST be a power of two.
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;


         // The number of entries in the table.
        private int size = 0;

         // The next size value at which to resize.
        private int threshold; // Default to 0

      
         // Set the resize threshold to maintain at worst a 2/3 load factor.  
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }

         // Increment i modulo len.
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }

        // Decrement i modulo len.
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }

        // ThreadLocalMap构造方法, 只有当我们的线程第一次有TheadLocal对象和数据要存储的时候才会调用
        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 < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal