Java基础快速入门: 并发工具类详解之Hashtable与ConcurrentHashMap原理、安全机制及代码实践

本文纲要

HashMap线程不安全演示
Hashtable线程安全与原理
ConcurrentHashMap基本使用
ConcurrentHashMap 1.7原理（分段锁）
ConcurrentHashMap 1.8原理（CAS + synchronized）
CountDownLatch使用场景与原理
Semaphore使用场景与原理

HashMap线程不安全演示

1 ）项目结构

text 复制代码

src/
└── com/
    └── wb/
        ├── mymap/
        │   ├── MyHashMapDemo.java
        │   ├── MyHashtableDemo.java
        │   └── MyConcurrentHashMapDemo.java
        ├── mycountdownlatch/
        │   ├── MotherThread.java
        │   ├── ChileThread1.java
        │   ├── ChileThread2.java
        │   ├── ChileThread3.java
        │   └── MyCountDownLatchDemo.java
        └── mysemaphore/
            ├── MyRunnable.java
            └── MySemaphoreDemo.java

在多线程环境下，HashMap 因为线程抢夺CPU的随机性，会出现数据安全问题。

例如两个线程同时 put 数据，可能导致某个键的值为 null。

java 复制代码

package com.wb.mymap;
 
import java.util.HashMap;
 
public class MyHashMapDemo {
    public static void main(String[] args) throws InterruptedException {
        HashMap<String, String> hm = new HashMap<>();
 
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 25; i++) {
                hm.put(i + "", i + "");
            }
        });
 
        Thread t2 = new Thread(() -> {
            for (int i = 25; i < 51; i++) {
                hm.put(i + "", i + "");
            }
        });
 
        t1.start();
        t2.start();
 
        System.out.println("----------------------------");
        // 为了t1和t2能把数据全部添加完毕 
        Thread.sleep(1000);
 
        // 预期输出: 0 1 2 3 ... 50 
        for (int i = 0; i < 51; i++) {
            System.out.println(hm.get(i + ""));
        }
    }
}

运行结果可能在某处打印 null，说明 HashMap 在多线程下不安全。

Hashtable线程安全与原理

Hashtable 可以保证线程安全，但效率极低，因为它在几乎所有方法上都加了 synchronized 关键字，采用悲观锁，操作时会将整张表锁住。

java 复制代码

package com.wb.mymap;
 
import java.util.Hashtable;
 
public class MyHashtableDemo {
    public static void main(String[] args) throws InterruptedException {
        Hashtable<String, String> hm = new Hashtable<>();
 
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 25; i++) {
                hm.put(i + "", i + "");
            }
        });
 
        Thread t2 = new Thread(() -> {
            for (int i = 25; i < 51; i++) {
                hm.put(i + "", i + "");
            }
        });
 
        t1.start();
        t2.start();
 
        System.out.println("----------------------------");
        Thread.sleep(1000);
 
        for (int i = 0; i < 51; i++) {
            System.out.println(hm.get(i + ""));
        }
    }
}

多次运行结果均完整，不会出现 null。

Hashtable锁机制示意
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锁住
等待
释放锁
线程1
整张表
线程2
线程2操作

小结： Hashtable 底层是哈希表结构（数组+链表），默认长度16，加载因子0.75。因为它对所有访问方法使用 synchronized，一旦有线程访问，整张表被锁，其他线程只能等待，效率低下。

ConcurrentHashMap基本使用

JDK1.5 提供了 ConcurrentHashMap，它既保证线程安全，又兼顾效率。它实现了 Map 接口，可以使用 put、keySet、entrySet 等方法。

java 复制代码

package com.wb.mymap;
 
import java.util.concurrent.ConcurrentHashMap;
 
public class MyConcurrentHashMapDemo {
    public static void main(String[] args) throws InterruptedException {
        ConcurrentHashMap<String, String> hm = new ConcurrentHashMap<>();
 
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 25; i++) {
                hm.put(i + "", i + "");
            }
        });
 
        Thread t2 = new Thread(() -> {
            for (int i = 25; i < 51; i++) {
                hm.put(i + "", i + "");
            }
        });
 
        t1.start();
        t2.start();
 
        System.out.println("----------------------------");
        Thread.sleep(1000);
 
        for (int i = 0; i < 51; i++) {
            System.out.println(hm.get(i + ""));
        }
    }
}

运行结果数据完整，无 null，同时效率高于 Hashtable。

ConcurrentHashMap 1.7原理

在JDK1.7中，ConcurrentHashMap 采用分段锁机制，内部由 Segment 数组和 HashEntry 数组组成。

1 ）创建对象

创建一个长度为16的 Segment 数组，加载因子0.75。
同时创建一个长度为2的小数组（HashEntry），并将其地址赋给 Segment[0]，其他索引为 null。该小数组作为模板。

2 ）添加元素流程

根据键的哈希值计算在 Segment 数组中的索引（第一次哈希）。
若该索引为 null，则按照模板创建新的 HashEntry 数组（默认长度2），并赋值给该索引。
进行二次哈希，计算元素在 HashEntry 数组中的位置。
若该位置为 null，直接存入；若不为 null，则调用 equals 比较，相同则覆盖，不同则形成链表（哈希桶）。
当 HashEntry 数组元素达到阈值（长度×0.75）时，会扩容为原来的2倍（Segment 数组本身不扩容）。

3 ）线程安全机制

每个 Segment 对象都有独立的锁，ConcurrentHashMap 通过对 Segment 加锁实现并发控制。
默认16个 Segment，最多允许16个线程同时操作不同的 Segment。

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ConcurrentHashMap1.7
锁住
锁住
等待S1
Segment数组
Segment0: HashEntry数组
Segment1: null
...
HashEntry
链表节点
链表节点
线程1
线程2
线程3

小结：

大数组（Segment）创建后不可扩容，小数组（HashEntry）可扩容2倍。
通过分段锁（ReentrantLock）保证线程安全，不同 Segment 可并发操作。

ConcurrentHashMap 1.8原理

JDK1.8改进了 ConcurrentHashMap，底层采用数组+链表+红黑树结构，与 HashMap 1.8类似，但使用 CAS + synchronized 保证线程安全。

1 ）初始化

空参构造什么也不做，延迟初始化，在第一次 put 时创建数组（默认容量16，加载因子0.75）。

2）添加元素流程

根据键的哈希值计算数组索引。
若该索引为 null，使用 CAS 算法直接将节点放入数组。
若不为 null，则使用 volatile 关键字获取该位置的最新节点，并采用 synchronized 锁住该节点（链表头节点或红黑树根节点）。
若是链表，遍历并比较 equals，相同则替换，不同则插入链表尾部；若链表长度 ≥ 8，转换为红黑树。
若是红黑树，按树的方式插入。

3 ）线程安全机制

不锁整张表，只锁当前操作的链表或红黑树头节点，其他线程可以同时操作其他桶。
使用 synchronized 代码块锁住头节点对象。

#mermaid-svg-MCYO4RKOhaeZZ8gz{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-MCYO4RKOhaeZZ8gz .error-icon{fill:#552222;}#mermaid-svg-MCYO4RKOhaeZZ8gz .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-MCYO4RKOhaeZZ8gz .marker{fill:#333333;stroke:#333333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .marker.cross{stroke:#333333;}#mermaid-svg-MCYO4RKOhaeZZ8gz svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-MCYO4RKOhaeZZ8gz p{margin:0;}#mermaid-svg-MCYO4RKOhaeZZ8gz .label{font-family:"trebuchet ms",verdana,arial,sans-serif;color:#333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .cluster-label text{fill:#333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .cluster-label span{color:#333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .cluster-label span p{background-color:transparent;}#mermaid-svg-MCYO4RKOhaeZZ8gz .label text,#mermaid-svg-MCYO4RKOhaeZZ8gz span{fill:#333;color:#333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .node rect,#mermaid-svg-MCYO4RKOhaeZZ8gz .node circle,#mermaid-svg-MCYO4RKOhaeZZ8gz .node ellipse,#mermaid-svg-MCYO4RKOhaeZZ8gz .node polygon,#mermaid-svg-MCYO4RKOhaeZZ8gz .node path{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#mermaid-svg-MCYO4RKOhaeZZ8gz .rough-node .label text,#mermaid-svg-MCYO4RKOhaeZZ8gz .node .label text,#mermaid-svg-MCYO4RKOhaeZZ8gz .image-shape .label,#mermaid-svg-MCYO4RKOhaeZZ8gz .icon-shape .label{text-anchor:middle;}#mermaid-svg-MCYO4RKOhaeZZ8gz .node .katex path{fill:#000;stroke:#000;stroke-width:1px;}#mermaid-svg-MCYO4RKOhaeZZ8gz .rough-node .label,#mermaid-svg-MCYO4RKOhaeZZ8gz .node .label,#mermaid-svg-MCYO4RKOhaeZZ8gz .image-shape .label,#mermaid-svg-MCYO4RKOhaeZZ8gz .icon-shape .label{text-align:center;}#mermaid-svg-MCYO4RKOhaeZZ8gz .node.clickable{cursor:pointer;}#mermaid-svg-MCYO4RKOhaeZZ8gz .root .anchor path{fill:#333333!important;stroke-width:0;stroke:#333333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .arrowheadPath{fill:#333333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edgePath .path{stroke:#333333;stroke-width:2.0px;}#mermaid-svg-MCYO4RKOhaeZZ8gz .flowchart-link{stroke:#333333;fill:none;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edgeLabel{background-color:rgba(232,232,232, 0.8);text-align:center;}#mermaid-svg-MCYO4RKOhaeZZ8gz .edgeLabel p{background-color:rgba(232,232,232, 0.8);}#mermaid-svg-MCYO4RKOhaeZZ8gz .edgeLabel rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#mermaid-svg-MCYO4RKOhaeZZ8gz .labelBkg{background-color:rgba(232, 232, 232, 0.5);}#mermaid-svg-MCYO4RKOhaeZZ8gz .cluster rect{fill:#ffffde;stroke:#aaaa33;stroke-width:1px;}#mermaid-svg-MCYO4RKOhaeZZ8gz .cluster text{fill:#333;}#mermaid-svg-MCYO4RKOhaeZZ8gz .cluster span{color:#333;}#mermaid-svg-MCYO4RKOhaeZZ8gz div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:12px;background:hsl(80, 100%, 96.2745098039%);border:1px solid #aaaa33;border-radius:2px;pointer-events:none;z-index:100;}#mermaid-svg-MCYO4RKOhaeZZ8gz .flowchartTitleText{text-anchor:middle;font-size:18px;fill:#333;}#mermaid-svg-MCYO4RKOhaeZZ8gz rect.text{fill:none;stroke-width:0;}#mermaid-svg-MCYO4RKOhaeZZ8gz .icon-shape,#mermaid-svg-MCYO4RKOhaeZZ8gz .image-shape{background-color:rgba(232,232,232, 0.8);text-align:center;}#mermaid-svg-MCYO4RKOhaeZZ8gz .icon-shape p,#mermaid-svg-MCYO4RKOhaeZZ8gz .image-shape p{background-color:rgba(232,232,232, 0.8);padding:2px;}#mermaid-svg-MCYO4RKOhaeZZ8gz .icon-shape .label rect,#mermaid-svg-MCYO4RKOhaeZZ8gz .image-shape .label rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#mermaid-svg-MCYO4RKOhaeZZ8gz .label-icon{display:inline-block;height:1em;overflow:visible;vertical-align:-0.125em;}#mermaid-svg-MCYO4RKOhaeZZ8gz .node .label-icon path{fill:currentColor;stroke:revert;stroke-width:revert;}#mermaid-svg-MCYO4RKOhaeZZ8gz :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} 锁机制
ConcurrentHashMap1.8
synchronized
synchronized
等待D
数组
桶0: 链表
桶1: 红黑树
节点1
节点2
根节点
子节点
线程1
线程2
线程3

小结：

空参构造延迟初始化，首次 put 创建数组。
通过 CAS 操作向空桶添加节点。
通过 synchronized 锁住桶头节点，实现细粒度并发控制。
链表长度 ≥ 8 时转为红黑树，提升查询效率。

CountDownLatch使用场景与原理

CountDownLatch 允许一条线程等待其他多条线程执行完毕后再执行。

经典场景：妈妈等待三个孩子吃完饺子后收拾碗筷。

1 ）核心方法

方法	说明
`CountDownLatch(int count)`	构造方法，count 为等待线程数量（底层定义计数器）
`await()`	让当前线程等待，直到计数器变为0
`countDown()`	每调用一次，计数器减1；当计数器为0时，唤醒等待线程

2 ）代码实现

java 复制代码

package com.wb.mycountdownlatch;
 
public class MotherThread extends Thread {
    private CountDownLatch countDownLatch;
    public MotherThread(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
 
    @Override
    public void run() {
        // 1.等待 
        try {
            // 当计数器变成0时，会自动唤醒这里等待的线程
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // 2.收拾碗筷
        System.out.println("妈妈在收拾碗筷");
    }
}

 
```java
package com.wb.mycountdownlatch;
 
public class ChileThread1 extends Thread {
    private CountDownLatch countDownLatch;
    public ChileThread1(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
 
    @Override
    public void run() {
        // 1.吃饺子
        for (int i = 1; i <= 10; i++) {
            System.out.println(getName() + "在吃第" + i + "个饺子");
        }
        // 2.吃完说一声
        // 每一次countDown方法的时候，就让计数器-1
        countDownLatch.countDown();
    }
}

其余孩子线程（ChileThread2、ChileThread3）类似，分别吃15个和20个饺子。

java 复制代码

package com.wb.mycountdownlatch;
 
import java.util.concurrent.CountDownLatch;
 
public class MyCountDownLatchDemo {
    public static void main(String[] args) {
        // 1.创建CountDownLatch对象，需要传递给四个线程。
        // 在底层定义了一个计数器，此时计数器的值为3
        CountDownLatch countDownLatch = new CountDownLatch(3);
        // 2.创建四个线程对象并开启
        MotherThread motherThread = new MotherThread(countDownLatch);
        motherThread.start();
 
        ChileThread1 t1 = new ChileThread1(countDownLatch);
        t1.setName("小明");
 
        ChileThread2 t2 = new ChileThread2(countDownLatch);
        t2.setName("小红");
 
        ChileThread3 t3 = new ChileThread3(countDownLatch);
        t3.setName("小刚");
 
        t1.start();
        t2.start();
        t3.start();
    }
}

3 ）执行流程
小刚小红小明妈妈线程小刚小红小明妈妈线程 #mermaid-svg-RtjWSeGntM2bPK0x{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-RtjWSeGntM2bPK0x .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-RtjWSeGntM2bPK0x .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-RtjWSeGntM2bPK0x .error-icon{fill:#552222;}#mermaid-svg-RtjWSeGntM2bPK0x .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-RtjWSeGntM2bPK0x .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-RtjWSeGntM2bPK0x .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-RtjWSeGntM2bPK0x .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-RtjWSeGntM2bPK0x .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-RtjWSeGntM2bPK0x .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-RtjWSeGntM2bPK0x .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-RtjWSeGntM2bPK0x .marker{fill:#333333;stroke:#333333;}#mermaid-svg-RtjWSeGntM2bPK0x .marker.cross{stroke:#333333;}#mermaid-svg-RtjWSeGntM2bPK0x svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-RtjWSeGntM2bPK0x p{margin:0;}#mermaid-svg-RtjWSeGntM2bPK0x .actor{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-RtjWSeGntM2bPK0x text.actor>tspan{fill:black;stroke:none;}#mermaid-svg-RtjWSeGntM2bPK0x .actor-line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-RtjWSeGntM2bPK0x .innerArc{stroke-width:1.5;stroke-dasharray:none;}#mermaid-svg-RtjWSeGntM2bPK0x .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333;}#mermaid-svg-RtjWSeGntM2bPK0x .messageLine1{stroke-width:1.5;stroke-dasharray:2,2;stroke:#333;}#mermaid-svg-RtjWSeGntM2bPK0x #arrowhead path{fill:#333;stroke:#333;}#mermaid-svg-RtjWSeGntM2bPK0x .sequenceNumber{fill:white;}#mermaid-svg-RtjWSeGntM2bPK0x #sequencenumber{fill:#333;}#mermaid-svg-RtjWSeGntM2bPK0x #crosshead path{fill:#333;stroke:#333;}#mermaid-svg-RtjWSeGntM2bPK0x .messageText{fill:#333;stroke:none;}#mermaid-svg-RtjWSeGntM2bPK0x .labelBox{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-RtjWSeGntM2bPK0x .labelText,#mermaid-svg-RtjWSeGntM2bPK0x .labelText>tspan{fill:black;stroke:none;}#mermaid-svg-RtjWSeGntM2bPK0x .loopText,#mermaid-svg-RtjWSeGntM2bPK0x .loopText>tspan{fill:black;stroke:none;}#mermaid-svg-RtjWSeGntM2bPK0x .loopLine{stroke-width:2px;stroke-dasharray:2,2;stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-RtjWSeGntM2bPK0x .note{stroke:#aaaa33;fill:#fff5ad;}#mermaid-svg-RtjWSeGntM2bPK0x .noteText,#mermaid-svg-RtjWSeGntM2bPK0x .noteText>tspan{fill:black;stroke:none;}#mermaid-svg-RtjWSeGntM2bPK0x .activation0{fill:#f4f4f4;stroke:#666;}#mermaid-svg-RtjWSeGntM2bPK0x .activation1{fill:#f4f4f4;stroke:#666;}#mermaid-svg-RtjWSeGntM2bPK0x .activation2{fill:#f4f4f4;stroke:#666;}#mermaid-svg-RtjWSeGntM2bPK0x .actorPopupMenu{position:absolute;}#mermaid-svg-RtjWSeGntM2bPK0x .actorPopupMenuPanel{position:absolute;fill:#ECECFF;box-shadow:0px 8px 16px 0px rgba(0,0,0,0.2);filter:drop-shadow(3px 5px 2px rgb(0 0 0 / 0.4));}#mermaid-svg-RtjWSeGntM2bPK0x .actor-man line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-RtjWSeGntM2bPK0x .actor-man circle,#mermaid-svg-RtjWSeGntM2bPK0x line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;stroke-width:2px;}#mermaid-svg-RtjWSeGntM2bPK0x :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} await() 等待吃饺子(10个)countDown() 计数器2吃饺子(15个)countDown() 计数器1吃饺子(20个)countDown() 计数器0被唤醒，收拾碗筷

小结： CountDownLatch 通过内部计数器实现线程等待，一次递减，计数器为0时唤醒等待线程，常用于主线程等待子线程完成的场景。

Semaphore使用场景与原理

Semaphore 用于控制同时访问特定资源的线程数量，如同单行道最多允许2辆车同时通行，发通行证的管理员。

1 ）核心方法

方法	说明
`Semaphore(int permits)`	构造方法，`permits` 为最大通行证数量（允许同时执行的线程数）
`acquire()`	获得通行证，若无则线程等待
`release()`	归还通行证，唤醒等待线程

2 ）代码实现

java 复制代码

package com.wb.mysemaphore;
 
import java.util.concurrent.Semaphore;
 
public class MyRunnable implements Runnable {
    // 1.获得管理员对象，最多允许2个线程同时执行
    private Semaphore semaphore = new Semaphore(2);
 
    @Override
    public void run() {
        // 2.获得通行证
        try {
            semaphore.acquire();
            // 3.开始行驶
            System.out.println("获得了通行证开始行驶");
            Thread.sleep(2000);
            System.out.println("归还通行证");
            // 4.归还通行证
            semaphore.release();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

 
java
package com.wb.mysemaphore;
 
public class MySemaphoreDemo {
    public static void main(String[] args) {
        MyRunnable mr = new MyRunnable();
 
        for (int i = 0; i < 100; i++) {
            new Thread(mr).start();
        }
    }
}

3 ）执行效果
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Semaphore允许2个通过
线程1执行
线程2执行
release
release
唤醒等待线程
新线程进入

运行结果：最多同时有2个线程打印"获得了通行证开始行驶"，执行完归还后，下一个线程才能进入。

总结

Semaphore 通过许可证数量控制并发数，使用时先 acquire() 获取许可，执行完毕 release() 归还，保证同时访问资源的线程数不超过指定值。

以上是 Java 并发工具类中 Hashtable、ConcurrentHashMap、CountDownLatch 和 Semaphore 的核心知识点，通过原理图解和代码示例帮助快速入门。