Android知识总结
一、分而治之原理（fork/join ）
十大经典算法：快速排序、推排序、归并排序、二叉排序、线性查找、深度优先、广度优先、Dijkstra、动态规化、朴素贝叶斯分类。
在计算机十大经典算法中，快速排序、归并排序和二分查找用的是分而治之原理。
1、定义
在Java的Fork/Join框架中，使用两个类完成上述操作
- 1、ForkJoinTask:我们要使用Fork/Join框架，首先需要创建一个ForkJoin任务。该类提供了在任务中执行fork和join的机制。通常情况下我们不需要直接集成ForkJoinTask类，只需要继承它的子类，Fork/Join框架提供了两个子类：
RecursiveTask的同步用法同时演示有返回结果，统计整形数组中所有元素的和。RecursiveAction的异步用法同时演示不要求返回值，遍历指定目标（含子目录）寻找指定类型文件。
- 2、ForkJoinPool:ForkJoinTask需要通过
ForkJoinPool来执行。- 任务分割出的子任务会添加到当前工作线程所维护的双端队列中，进入队列的头部。当一个工作线程的队列里暂时没有任务时，它会随机从其他工作线程的队列的尾部获取一个任务(
工作窃取算法)。
2、归并排序-同步用法
2.1、数组集合
public class MakeArray {
public static final int MAX_COUNT = 40000;
public static int[] getArrays(){
int[] nums = new int[MAX_COUNT];
Random random = new Random();
for (int i = 0; i < MAX_COUNT; i++) {
nums[i] = random.nextInt(MAX_COUNT);
}
return nums;
}
}
2.2、求数组中的和
public class SunArray {
public static class SumTask extends RecursiveTask{
private static final int THRESHOLD = MakeArray.MAX_COUNT/10;
private int[] nums;
private int fromIndex;
private int toIndex;
public SumTask(int[] nums, int fromIndex, int toIndex) {
this.nums = nums;
this.fromIndex = fromIndex;
this.toIndex = toIndex;
}
@Override
protected Integer compute() { //运用递归算法
if (toIndex - fromIndex < THRESHOLD){
System.out.println("form index = " + fromIndex + "toIndex = " + toIndex);
int count = 0;
for (int i = fromIndex; i < toIndex; i++) {
count += nums[i];
}
return count;
} else {
int mid = (toIndex + fromIndex) / 2;
SumTask left = new SumTask(nums, fromIndex, mid);
SumTask right = new SumTask(nums, mid, toIndex);
invokeAll(left, right);
return left.join() + right.join();
}
}
}
public static void main(String[] argc){
int[] arrays = MakeArray.getArrays();
ForkJoinPool forkJoinPool = new ForkJoinPool();
SumTask sumTask = new SumTask(arrays, 0, arrays.length);
long start = System.currentTimeMillis();
forkJoinPool.invoke(sumTask);
System.out.println("The count is" + sumTask.join() +
"spend time" + (System.currentTimeMillis() - start) + "ms");
}
}
3、异步用法
/**
*类说明：遍历指定目录（含子目录）找寻指定类型文件
*/
public class FindDirsFiles extends RecursiveAction {
private File path;
public FindDirsFiles(File path) {
this.path = path;
}
@Override
protected void compute() {
List subTasks = new ArrayList<>();
File[] files = path.listFiles();
if (files!=null){
for (File file : files) {
if (file.isDirectory()) {
// 对每个子目录都新建一个子任务。
subTasks.add(new FindDirsFiles(file));
} else {
// 遇到文件，检查。
if (file.getAbsolutePath().endsWith("txt")){
System.out.println("文件:" + file.getAbsolutePath());
}
}
}
if (!subTasks.isEmpty()) {
// 在当前的 ForkJoinPool 上调度所有的子任务。
for (FindDirsFiles subTask : invokeAll(subTasks)) {
subTask.join();
}
}
}
}
public static void main(String [] args){
try {
// 用一个 ForkJoinPool 实例调度总任务
ForkJoinPool pool = new ForkJoinPool();
FindDirsFiles task = new FindDirsFiles(new File("F:/"));
/*异步提交*/
pool.execute(task);
/*主线程做自己的业务工作*/
System.out.println("Task is Running......");
Thread.sleep(1);
int otherWork = 0;
for(int i=0;i<100;i++){
otherWork = otherWork+i;
}
System.out.println("Main Thread done sth......,otherWork=" +otherWork);
task.join();//阻塞方法
System.out.println("Task end");
} catch (Exception e) {
e.printStackTrace();
}
}
}
二、CountDownLatch 计数器
CountDownLatch 示意图
- countDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。
- 是通过一个计数器来实现的，计数器的初始值是线程的数量。每当一个线程执行完毕后，计数器的值就-1，当计数器的值为0时，闭锁上等待的线程就可以恢复工作了。
- 使用AQS的共享方式，内部实现了
AbstractQueuedSynchronizer的内部类。
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c - 1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
注意：一个线程可以多次减一；闭锁线程可以有多个且闭锁线程执行任务时其他线程可能还在执行
示例演示
/**
*类说明：演示CountDownLatch用法，
* 共5个初始化子线程，6个闭锁扣除点，扣除完毕后，主线程和业务线程才能继续执行
*/
public class UseCountDownLatch {
static CountDownLatch latch = new CountDownLatch(6);
/*初始化线程*/
private static class MyRunnable implements Runnable {
@Override
public void run() {
System.out.println("Thread_" + Thread.currentThread().getId()
+ " ready init work......");
latch.countDown();
for (int i = 0; i < 2; i++) {
System.out.println("Thread_" + Thread.currentThread().getId()
+ " ........continue do its work");
}
}
}
/*业务线程等待latch的计数器为0完成*/
private static class MyThread implements Runnable {
@Override
public void run() {
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int i = 0; i < 3; i++) {
System.out.println("BusiThread_" + Thread.currentThread().getId()
+ " do business-----");
}
}
}
public static void main(String[] args) throws InterruptedException {
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(1);
System.out.println("Thread_" + Thread.currentThread().getId()
+ " ready init work step 1st......");
latch.countDown();
System.out.println("begin step 2nd.......");
Thread.sleep(1);
System.out.println("Thread_" + Thread.currentThread().getId()
+ " ready init work step 2nd......");
latch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
new Thread(new MyThread()).start();
for (int i = 0; i <= 3; i++) {
Thread thread = new Thread(new MyRunnable());
thread.start();
}
latch.await();
System.out.println("Main do ites work........");
}
}
三、CyclicBarrier
- CyclicBarrier 可以反复的调用，可理解为
循环栅栏。 - CyclicBarrier是有工作线程本身协调完成的，CountDownLatch 工作线程是由外面的线程协调完成的。
- CyclicBarrier 在工作线程之后可以用 barrierAction 来完成汇总的；CountDownLatch 在运行时不能做其他的操作的。。
- CyclicBarrier 是参与线程的个数是相等的，CountDownLatch 可以不相同。
CyclicBarrier 示意图
- 示例1
public class UseCyclicBarrier {
public static CyclicBarrier barrier = new CyclicBarrier(4, new CollectThread());
//存放子线程工作结果的容器
private static ConcurrentHashMap resultMap = new ConcurrentHashMap<>();
public static void main(String[] args) {
for (int i = 0; i <= 4; i++) {
Thread thread = new Thread(new SubThread());
thread.start();
}
}
private static class CollectThread implements Runnable {
@Override
public void run() {
StringBuilder result = new StringBuilder();
for (Map.Entry workResult : resultMap.entrySet()) {
result.append("[" + workResult.getValue() + "]");
}
System.out.println(" the result = " + result);
System.out.println("do other business........");
}
}
private static class SubThread implements Runnable {
@Override
public void run() {
long id = Thread.currentThread().getId();
resultMap.put(Thread.currentThread().getId() + "", id);
Random r = new Random();
try {
Thread.sleep(1000 + id);
System.out.println("Thread_" + id + " ....do something ");
barrier.await();
Thread.sleep(1000 + id);
barrier.await(); //可以反复的调用
System.out.println("Thread_" + id + " ....do its business ");
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
- 示例2
public class CyclicBarrierDemo {
public static class Soldier implements Runnable {
private String soldier;
private final CyclicBarrier cyclic;
Soldier(CyclicBarrier cyclic, String soldierName) {
this.cyclic = cyclic;
this.soldier = soldierName;
}
public void run() {
try {
//等待所有士兵到齐
cyclic.await();
doWork();
//等待所有士兵完成工作
cyclic.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
void doWork() {
try {
Thread.sleep(Math.abs(new Random().nextInt() % 10000));
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(soldier + ":任务完成");
}
}
public static class BarrierRun implements Runnable {
boolean flag;
int N;
public BarrierRun(boolean flag, int N) {
this.flag = flag;
this.N = N;
}
public void run() {
if (flag) {
System.out.println("司令:[士兵" + N + "个，任务完成！]");
} else {
System.out.println("司令:[士兵" + N + "个，集合完毕！]");
flag = true;
}
}
}
public static void main(String args[]) throws InterruptedException {
final int N = 10;
Thread[] allSoldier = new Thread[N];
boolean flag = false;
CyclicBarrier cyclic = new CyclicBarrier(N, new BarrierRun(flag, N));
//设置屏障点，主要是为了执行这个方法
System.out.println("集合队伍！");
for (int i = 0; i < N; ++i) {
System.out.println("士兵 " + i + " 报道！");
allSoldier[i] = new Thread(new Soldier(cyclic, "士兵 " + i));
allSoldier[i].start();
}
}
}
四、Semaphore 信号量
Semaphore 可以通过其限制执行的线程数量，达到限流（流控）的效果。
当一个线程执行时先通过其方法进行获取许可操作，获取到许可的线程继续执行业务逻辑，当线程执行完成后进行释放许可操作，未获取达到许可的线程进行等待或者直接结束。
- 示例1
private static class MyRunnable implements Runnable {
// 成员属性 Semaphore对象
private final Semaphore semaphore;
public MyRunnable(Semaphore semaphore) {
this.semaphore = semaphore;
}
public void run() {
String threadName = Thread.currentThread().getName();
// 获取许可
boolean acquire = semaphore.tryAcquire();
// 未获取到许可 结束
if (!acquire) {
System.out.println("线程【" + threadName + "】未获取到许可，结束");
return;
}
// 获取到许可
try {
System.out.println("线程【" + threadName + "】获取到许可");
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// 释放许可
semaphore.release();
System.out.println("线程【" + threadName + "】释放许可");
}
}
}
public static void main(String[] args) {
Semaphore semaphore = new Semaphore(2);
for (int i = 0; i <= 10; i ++) {
MyRunnable runnable = new MyRunnable(semaphore);
Thread thread = new Thread(runnable, "Thread-" + i);
thread.start();
}
}
- 示例2-数据库连接池
//数据库连接实现
public class SqlConnectImpl implements Connection{
/*拿一个数据库连接*/
public static final Connection fetchConnection(){
return new SqlConnectImpl();
}
...
}
//一个数据库连接池的实现
public class DBPoolSemaphore {
private final static int POOL_SIZE = 10;
//两个指示器，分别表示池子还有可用连接和已用连接
private final Semaphore useful, useless;
//存放数据库连接的容器
private static LinkedList pool = new LinkedList();
//初始化池
static {
for (int i = 0; i < POOL_SIZE; i++) {
pool.addLast(SqlConnectImpl.fetchConnection());
}
}
public DBPoolSemaphore() {
this.useful = new Semaphore(10);
this.useless = new Semaphore(0);
}
/*归还连接*/
public void returnConnect(Connection connection) throws InterruptedException {
if(connection!=null) {
System.out.println("当前有"+useful.getQueueLength()+"个线程等待数据库连接!!"
+"可用连接数："+useful.availablePermits());
useless.acquire();
synchronized (pool) {
pool.addLast(connection);
}
useful.release();
}
}
/*从池子拿连接*/
public Connection takeConnect() throws InterruptedException {
useful.acquire();
Connection connection;
synchronized (pool) {
connection = pool.removeFirst();
}
useless.release();
return connection;
}
}
//测试类
public class AppTest {
private static DBPoolSemaphore dbPool = new DBPoolSemaphore();
private static class BusiThread extends Thread{
@Override
public void run() {
Random r = new Random();//让每个线程持有连接的时间不一样
long start = System.currentTimeMillis();
try {
Connection connect = dbPool.takeConnect();
System.out.println("Thread_"+Thread.currentThread().getId()
+"_获取数据库连接共耗时【"+(System.currentTimeMillis()-start)+"】ms.");
SleepTools.ms(100+r.nextInt(100));//模拟业务操作，线程持有连接查询数据
System.out.println("查询数据完成，归还连接！");
dbPool.returnConnect(connect);
} catch (InterruptedException e) {
}
}
}
public static void main(String[] args) {
for (int i = 0; i < 50; i++) {
Thread thread = new BusiThread();
thread.start();
}
}
}
五、Exchange
主要用于两个线程间的数据交换
public class UseExchange {
private static final Exchanger> exchange = new Exchanger>();
public static void main(String[] args) {
new Thread(new Runnable() {
@Override
public void run() {
Set setA = new HashSet();//存放数据的容器
try {
//添加数据
setA.add("dataA");
setA = exchange.exchange(setA);//交换set
/*处理交换后的数据*/
for (String str : setA) {
System.out.println(Thread.currentThread() + " A :" + str);
}
System.out.println(Thread.currentThread() + " A :" + exchange);
} catch (InterruptedException e) {
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
Set setB = new HashSet();//存放数据的容器
try {
//添加数据
setB.add("dataB1");
setB.add("dataB2");
setB = exchange.exchange(setB);//交换set
/*处理交换后的数据*/
for (String str : setB) {
System.out.println(Thread.currentThread() + " B :" + str);
}
System.out.println(Thread.currentThread() + " B :" + exchange);
} catch (InterruptedException e) {
}
}
}).start();
}
}
六、Callable、 Future和FutureTask
public class UseFuture {
/*实现Callable接口，允许有返回值*/
private static class UseCallable implements Callable {
private int sum;
@Override
public Integer call() throws Exception {
System.out.println("Callable子线程开始计算！");
Thread.sleep(2000);
for (int i = 0; i < 5000; i++) {
sum = sum + i;
}
System.out.println("Callable子线程计算结束！结果为: " + sum);
return sum;
}
}
public static void main(String[] args)
throws InterruptedException, ExecutionException {
UseCallable useCallable = new UseCallable();
FutureTask futureTask //用FutureTask包装Callable
= new FutureTask<>(useCallable);
new Thread(futureTask).start();//交给Thread去运行
Random r = new Random();
Thread.sleep(1000);
if (r.nextBoolean()) {//用随机的方式决定是获得结果还是终止任务
System.out.println("Get UseCallable result = " + futureTask.get());
} else {
System.out.println("中断计算。 ");
futureTask.cancel(true);
}
}
}