Springboot 中RedisTemplate使用scan来获取所有的key底层做了哪些事情

直接上代码，围绕着代码来讨论

redisTemplate.execute((RedisCallback<Object>)(connection)->{Cursor<byte[]> scan = connection
            .scan(ScanOptions.scanOptions().count(2).match("*").build());
    scan.forEachRemaining((bytes)->{System.out.println(newString(bytes));});//while (scan.hasNext()) {//    System.out.println(new String(scan.next()));//}returnnull;});

通过上述代码我们可以看到，可以通过迭代器来获取到redis中单库中所有的key，但这样底层是怎么做的呢？我们现在假定有两种方案去实现这种方案。

① redis 客户端（jedis、lettuce）scan执行之后返回了所有的数据，迭代器next()每次都从数据集中取出一条消费。但这样有个疑点 =>

ScanOptions.scanOptions().count(2).match("*").build()

我们知道count(2)代表着每次只向redis中请求2条返回数据呀。如果全部返回也不符合scan命令的设计！

② 每次迭代next()都要判断是否数据集中还有数据，没有的话去redis中通过游标取下次一的数据集（2条）。然后将获取到数据集迭代器替换到游标中，上一个数据集回收（防止内存过大），使迭代器可以正常流转。

那上面两条都是我自己的猜测，可能两条都不对，也有可能两条对一条，具体我们还是要看源码怎么做的。

分析源码

Cursor<byte[]> scan = connection.scan(ScanOptions.scanOptions().count(2).match("*").build());

这段代码返回了一个Cursor 游标，我们看看他的具体实现是这个类：

org.springframework.data.redis.core.ScanCursor

。
这个类实现了Cursor,实现了迭代器的所有方法。

当我们执行

forEachRemaining

或者

hasNext()

的时候都会去判断

@OverridepublicbooleanhasNext(){assertCursorIsOpen();// delegate可以理解为数据集的迭代器，是一个list类型，从redis获取到的数据集会放到这个list中while(!delegate.hasNext()&&!CursorState.FINISHED.equals(state)){// 可以看到如果说数据集读取完了，会继续去往redis中取下一组scan(cursorId);}if(delegate.hasNext()){returntrue;}return cursorId >0;}

scan 方法

privatevoidscan(long cursorId){// 去请求redis获取下一组keyScanIteration<T> result =doScan(cursorId,this.scanOptions);// 将数据集processScanResult(result);}

processScanResult 方法

privatevoidprocessScanResult(ScanIteration<T> result){// redis 中的游标
    cursorId = result.getCursorId();if(isFinished(cursorId)){
        state =CursorState.FINISHED;}if(!CollectionUtils.isEmpty(result.getItems())){// 替换迭代器 *** 重要代码
        delegate = result.iterator();}else{resetDelegate();}}

可以看到forEachRemaining其实也要去先判断数据集中有值。

自此我们验证了上述的两条推断中第二条是争取的，每次迭代器流转的时候都会去判断是否还有数据，没有的话就会从redis取到新的数据集，然后把数据集的迭代器替换到游标中，从而实现了redis单库取出所有的key。spring-data-redis中实现的很优雅，很巧妙，我们也可以学习这种设计模式来批量获取远程分页数据等。最后我们来看下ScanCursor的整体代码：

/*
 * Copyright 2014-2022 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */packageorg.springframework.data.redis.core;importjava.util.Collections;importjava.util.Iterator;importjava.util.NoSuchElementException;importorg.springframework.dao.InvalidDataAccessApiUsageException;importorg.springframework.lang.Nullable;importorg.springframework.util.Assert;importorg.springframework.util.CollectionUtils;/**
 * Redis client agnostic {@link Cursor} implementation continuously loading additional results from Redis server until
 * reaching its starting point {@code zero}. <br />
 * <strong>Note:</strong> Please note that the {@link ScanCursor} has to be initialized ({@link #open()} prior to usage.
 *
 * @author Christoph Strobl
 * @author Thomas Darimont
 * @author Duobiao Ou
 * @author Marl Paluch
 * @param <T>
 * @since 1.4
 */publicabstractclassScanCursor<T>implementsCursor<T>{privateCursorState state;// 游标状态privatelong cursorId;// 游标当前位置privateIterator<T> delegate;// 从redis获取到的结果集的迭代器，只要实现过Iterator的类都可以被代理privatefinalScanOptions scanOptions;// 配置信息privatelong position;// 位置/**
     * Crates new {@link ScanCursor} with {@code id=0} and {@link ScanOptions#NONE}
     */publicScanCursor(){this(ScanOptions.NONE);}/**
     * Crates new {@link ScanCursor} with {@code id=0}.
     *
     * @param options the scan options to apply.
     */publicScanCursor(ScanOptions options){this(0, options);}/**
     * Crates new {@link ScanCursor} with {@link ScanOptions#NONE}
     *
     * @param cursorId the cursor Id.
     */publicScanCursor(long cursorId){this(cursorId,ScanOptions.NONE);}/**
     * Crates new {@link ScanCursor}
     *
     * @param cursorId the cursor Id.
     * @param options Defaulted to {@link ScanOptions#NONE} if {@code null}.
     */publicScanCursor(long cursorId,@NullableScanOptions options){this.scanOptions = options !=null? options :ScanOptions.NONE;this.cursorId = cursorId;this.state =CursorState.READY;this.delegate =Collections.emptyIterator();}privatevoidscan(long cursorId){ScanIteration<T> result =doScan(cursorId,this.scanOptions);processScanResult(result);}/**
     * Performs the actual scan command using the native client implementation. The given {@literal options} are never
     * {@code null}.
     *
     * @param cursorId
     * @param options
     * @return
     */protectedabstractScanIteration<T>doScan(long cursorId,ScanOptions options);/**
     * Initialize the {@link Cursor} prior to usage.
     */publicfinalScanCursor<T>open(){if(!isReady()){thrownewInvalidDataAccessApiUsageException("Cursor already "+ state +". Cannot (re)open it.");}

        state =CursorState.OPEN;doOpen(cursorId);returnthis;}/**
     * Customization hook when calling {@link #open()}.
     *
     * @param cursorId
     */protectedvoiddoOpen(long cursorId){scan(cursorId);}privatevoidprocessScanResult(ScanIteration<T> result){

        cursorId = result.getCursorId();if(isFinished(cursorId)){
            state =CursorState.FINISHED;}if(!CollectionUtils.isEmpty(result.getItems())){
            delegate = result.iterator();}else{resetDelegate();}}/**
     * Check whether {@code cursorId} is finished.
     *
     * @param cursorId the cursor Id
     * @return {@literal true} if the cursor is considered finished, {@literal false} otherwise.s
     * @since 2.1
     */protectedbooleanisFinished(long cursorId){return cursorId ==0;}privatevoidresetDelegate(){
        delegate =Collections.emptyIterator();}/*
     * (non-Javadoc)
     * @see org.springframework.data.redis.core.Cursor#getCursorId()
     */@OverridepubliclonggetCursorId(){return cursorId;}/*
     * (non-Javadoc)
     * @see java.util.Iterator#hasNext()
     */@OverridepublicbooleanhasNext(){assertCursorIsOpen();while(!delegate.hasNext()&&!CursorState.FINISHED.equals(state)){scan(cursorId);}if(delegate.hasNext()){returntrue;}return cursorId >0;}privatevoidassertCursorIsOpen(){if(isReady()||isClosed()){thrownewInvalidDataAccessApiUsageException("Cannot access closed cursor. Did you forget to call open()?");}}/*
     * (non-Javadoc)
     * @see java.util.Iterator#next()
     */@OverridepublicTnext(){assertCursorIsOpen();if(!hasNext()){thrownewNoSuchElementException("No more elements available for cursor "+ cursorId +".");}T next =moveNext(delegate);
        position++;return next;}/**
     * Fetch the next item from the underlying {@link Iterable}.
     *
     * @param source
     * @return
     */protectedTmoveNext(Iterator<T> source){return source.next();}/*
     * (non-Javadoc)
     * @see java.util.Iterator#remove()
     */@Overridepublicvoidremove(){thrownewUnsupportedOperationException("Remove is not supported");}/*
     * (non-Javadoc)
     * @see java.io.Closeable#close()
     */@Overridepublicfinalvoidclose(){try{doClose();}finally{
            state =CursorState.CLOSED;}}/**
     * Customization hook for cleaning up resources on when calling {@link #close()}.
     */protectedvoiddoClose(){}/*
     * (non-Javadoc)
     * @see org.springframework.data.redis.core.Cursor#isClosed()
     */@OverridepublicbooleanisClosed(){return state ==CursorState.CLOSED;}protectedfinalbooleanisReady(){return state ==CursorState.READY;}protectedfinalbooleanisOpen(){return state ==CursorState.OPEN;}/*
     * (non-Javadoc)
     * @see org.springframework.data.redis.core.Cursor#getPosition()
     */@OverridepubliclonggetPosition(){return position;}/**
     * @author Thomas Darimont
     */enumCursorState{READY,OPEN,FINISHED,CLOSED;}}