2020了你還不會Java8新特性？（四）Collector類源碼分析
Collector類源碼分析
jdk8是怎麼對底層完成支持的。不瞭解底層，平時用還能夠，可是遇到問題的時候就會卡在那裏。遲遲滅有解決方案。在學習一門新技術時，先學習怎麼去用，不要執着於源碼。可是隨着用的愈來愈多，你去了解底層是比較好的一種學習方法。html
有多種方法能夠實現同一個功能.什麼方式更好呢? 越具體的方法越好. 減小自動裝箱拆箱操做
- collect : 收集器
- Collector做爲collect方法的參數。
- Collector做爲一個接口。它是一個可變的匯聚操做，將輸入元素累計到一個可變的結果容器中；它會在全部元素都處理完畢後將累計的結果做爲一個最終的表示（這是一個可選操做）；它支持串行與並行兩種方式執行。（並非說並行必定比串行快。）
- Collects自己提供了關於Collectoe的常見匯聚實現，Collectors自己其實是一個工廠。
- 爲了確保串行和並行的結果一致，須要進行額外的處理。必需要知足兩個約束。
identity 同一性
associativity 結合性 - 同一性：對於任何一條並行線路來講 ，須要知足a == combiner.apply(a, supplier.get())。舉例來講：
（Listlist1,List list2 -> {list1.addAll(list2);return list1}）
結合性： 下方有舉例。
Collector收集器的實現源碼詳解
/**
* A <a href="package-summary.html#Reduction">mutable reduction operation</a> that
* accumulates input elements into a mutable result container, optionally transforming
* the accumulated result into a final representation after all input elements
* have been processed. Reduction operations can be performed either sequentially
* or in parallel.
Collector做爲一個接口。它是一個可變的匯聚操做，將輸入元素累計到一個可變的結果容器中；它會在全部元素都處理 完畢後將累計的結果做爲一個最終的表示（這是一個可選操做）；它支持串行與並行兩種方式執行。（並非說並行必定比串行快。）
* <p>Examples of mutable reduction operations include:
* accumulating elements into a {@code Collection}; concatenating
* strings using a {@code StringBuilder}; computing summary information about
* elements such as sum, min, max, or average; computing "pivot table" summaries
* such as "maximum valued transaction by seller", etc. The class {@link Collectors}
* provides implementations of many common mutable reductions.
Collects自己提供了關於Collectoe的常見匯聚實現，Collectors自己其實是一個工廠。
* <p>A {@code Collector} is specified by four functions that work together to
* accumulate entries into a mutable result container, and optionally perform
* a final transform on the result. They are: <ul>
* <li>creation of a new result container ({@link #supplier()})</li>
* <li>incorporating a new data element into a result container ({@link #accumulator()})</li>
* <li>combining two result containers into one ({@link #combiner()})</li>
* <li>performing an optional final transform on the container ({@link #finisher()})</li>
* </ul>
Collector 包含了4個參數
* <p>Collectors also have a set of characteristics, such as
* {@link Characteristics#CONCURRENT}, that provide hints that can be used by a
* reduction implementation to provide better performance.
*
* <p>A sequential implementation of a reduction using a collector would
* create a single result container using the supplier function, and invoke the
* accumulator function once for each input element. A parallel implementation
* would partition the input, create a result container for each partition,
* accumulate the contents of each partition into a subresult for that partition,
* and then use the combiner function to merge the subresults into a combined
* result.
舉例說明：
1，2， 3， 4 四個部分結果。
1，2 -》 5
5，3 -》 6
6，4 -》 6
### 同一性和結合性的解析：
* <p>To ensure that sequential and parallel executions produce equivalent
* results, the collector functions must satisfy an <em>identity</em> and an
* <a href="package-summary.html#Associativity">associativity</a> constraints.
爲了確保串行和並行的結果一致，須要進行額外的處理。必需要知足兩個約束。
identity 同一性
associativity 結合性
* <p>The identity constraint says that for any partially accumulated result,
* combining it with an empty result container must produce an equivalent
* result. That is, for a partially accumulated result {@code a} that is the
* result of any series of accumulator and combiner invocations, {@code a} must
* be equivalent to {@code combiner.apply(a, supplier.get())}.
同一性： 對於任何一條並行線路來講,須要知足a == combiner.apply(a, supplier.get())
* <p>The associativity constraint says that splitting the computation must
* produce an equivalent result. That is, for any input elements {@code t1}
* and {@code t2}, the results {@code r1} and {@code r2} in the computation
* below must be equivalent:
* <pre>{@code
* A a1 = supplier.get(); 串行：
* accumulator.accept(a1, t1); 第一個參數，每次累加的中間結果。 第二個參數，下一個要處理的參數
* accumulator.accept(a1, t2);
* R r1 = finisher.apply(a1); // result without splitting
*
* A a2 = supplier.get(); 並行：
* accumulator.accept(a2, t1); 第一個參數，每次累加的中間結果。 第二個參數，下一個要處理的參數
* A a3 = supplier.get();
* accumulator.accept(a3, t2);
* R r2 = finisher.apply(combiner.apply(a2, a3)); // result with splitting
* } </pre>
結合性： 如上例。 最終要求 r1 == r2
* <p>For collectors that do not have the {@code UNORDERED} characteristic,
* two accumulated results {@code a1} and {@code a2} are equivalent if
* {@code finisher.apply(a1).equals(finisher.apply(a2))}. For unordered
* collectors, equivalence is relaxed to allow for non-equality related to
* differences in order. (For example, an unordered collector that accumulated
* elements to a {@code List} would consider two lists equivalent if they
* contained the same elements, ignoring order.)
對於無序的收集器來講，等價性就被放鬆了，會考慮到順序上的區別對應的不相等性。
兩個集合中包含了相同的元素，可是忽略了順序。這種狀況下兩個的集合也是等價的。
### collector複合與注意事項：
* <p>Libraries that implement reduction （匯聚） based on {@code Collector}, such as
* {@link Stream#collect(Collector)}, must adhere to the following constraints:
* <ul>
* <li>The first argument passed to the accumulator function, both
* arguments passed to the combiner function, and the argument passed to the
* finisher function must be the result of a previous invocation of the
* result supplier, accumulator, or combiner functions.</li>
* <li>The implementation should not do anything with the result of any of
* the result supplier, accumulator, or combiner functions other than to
* pass them again to the accumulator, combiner, or finisher functions,
* or return them to the caller of the reduction operation.</li>
具體的實現來講，不該該對中間返回的結果進行額外的操做。除了最終的返回的結果。
* <li>If a result is passed to the combiner or finisher
* function, and the same object is not returned from that function, it is
* never used again.</li>
若是一個結果被傳遞給combiner or finisher，可是並無返回一個你傳遞的對象，說明你生成了一個新的結果或者建立了新的對象。這個結果就不會再被使用了。
* <li>Once a result is passed to the combiner or finisher function, it
* is never passed to the accumulator function again.</li>
一旦一個結果被傳遞給了 combiner or finisher 函數，他就不會再被傳遞給了accumulator函數了。
* <li>For non-concurrent collectors, any result returned from the result
* supplier, accumulator, or combiner functions must be serially
* thread-confined. This enables collection to occur in parallel without
* the {@code Collector} needing to implement any additional synchronization.
* The reduction implementation must manage that the input is properly
* partitioned, that partitions are processed in isolation, and combining
* happens only after accumulation is complete.</li>
線程和線程之間的處理都是獨立的，最終結束時再進行合併。
* <li>For concurrent collectors, an implementation is free to (but not
* required to) implement reduction concurrently. A concurrent reduction
* is one where the accumulator function is called concurrently from
* multiple threads, using the same concurrently-modifiable result container,
* rather than keeping the result isolated during accumulation.
* A concurrent reduction should only be applied if the collector has the
* {@link Characteristics#UNORDERED} characteristics or if the
* originating data is unordered.</li>
若是不是併發收集器，4個線程會生成4箇中間結果。
是併發收集器的話，4個線程會同時調用一個結果容器。
* </ul>
*
* <p>In addition to the predefined implementations in {@link Collectors}, the
* static factory methods {@link #of(Supplier, BiConsumer, BinaryOperator, Characteristics...)}
* can be used to construct collectors. For example, you could create a collector
* that accumulates widgets into a {@code TreeSet} with:
*
* <pre>{@code
* Collector<Widget, ?, TreeSet<Widget>> intoSet =
* Collector.of(TreeSet::new, TreeSet::add,
* (left, right) -> { left.addAll(right); return left; });
* }</pre>
經過Collector.of（傳進一個新的要操做的元素，結果容器處理的步驟，多線程處理的操做）
將流中的每一個Widget 添加到TreeSet中
* (This behavior is also implemented by the predefined collector
* {@link Collectors#toCollection(Supplier)}).
*
* @apiNote
* Performing a reduction operation with a {@code Collector} should produce a
* result equivalent to:
* <pre>{@code
* R container = collector.supplier().get();
* for (T t : data)
* collector.accumulator().accept(container, t);
* return collector.finisher().apply(container);
* }</pre>
api的說明： collector的finisher匯聚的實現過程。
* <p>However, the library is free to partition the input, perform the reduction
* on the partitions, and then use the combiner function to combine the partial
* results to achieve a parallel reduction. (Depending on the specific reduction
* operation, this may perform better or worse, depending on the relative cost
* of the accumulator and combiner functions.)
性能取決於accumulator and combiner的代價。 也就是說 並行流 並不必定比串行流效率高。
* <p>Collectors are designed to be <em>composed</em>; many of the methods
* in {@link Collectors} are functions that take a collector and produce
* a new collector. For example, given the following collector that computes
* the sum of the salaries of a stream of employees:
* <pre>{@code
* Collector<Employee, ?, Integer> summingSalaries
* = Collectors.summingInt(Employee::getSalary))
* }</pre>
蒐集器是能夠組合的： take a collector and produce a new collector.
蒐集器的實現過程。 如 員工的工資的求和。
* If we wanted to create a collector to tabulate the sum of salaries by
* department, we could reuse the "sum of salaries" logic using
* {@link Collectors#groupingBy(Function, Collector)}:
* <pre>{@code
* Collector<Employee, ?, Map<Department, Integer>> summingSalariesByDept
* = Collectors.groupingBy(Employee::getDepartment, summingSalaries);
* }</pre>
若是咱們想要新建一個蒐集器，咱們能夠複用以前的蒐集器。
實現過程。
* @see Stream#collect(Collector)
* @see Collectors
*
* @param <T> the type of input elements to the reduction operation
<T> 表明 流中的每個元素的類型。
* @param <A> the mutable accumulation type of the reduction operation (often
* hidden as an implementation detail)
<A> 表明 reduction操做的可變容器的類型。表示中間操做生成的結果的類型（如ArrayList）。
* @param <R> the result type of the reduction operation
<R> 表明 結果類型
* @since 1.8
*/
public interface Collector<T, A, R>{
/**
* A function that creates and returns a new mutable result container.
* A就表明每一次返回結果的類型
* @return a function which returns a new, mutable result container
*/
Supplier<A> supplier(); // 提供一個結果容器
/**
* A function that folds a value into a mutable result container.
* A表明中間操做返回結果的類型。 T是下一個代操做的元素的類型。
* @return a function which folds a value into a mutable result container
*/
BiConsumer<A, T> accumulator(); //不斷的向結果容器中添加元素。
/**
* A function that accepts two partial results and merges them. The
* combiner function may fold state from one argument into the other and
* return that, or may return a new result container.
* A 中間操做返回結果的類型。
* @return a function which combines two partial results into a combined
* result
*/
BinaryOperator<A> combiner(); //在多線程中 合併 部分結果。
/**
和並行流緊密相關的
接收兩個結果，將兩個部分結果合併到一塊兒。
combiner函數，有4個線程同時去執行，那麼就會有生成4個部分結果。
舉例說明：
1，2， 3， 4 四個部分結果。
1，2 -》 5
5，3 -》 6
6，4 -》 6
1,2合併返回5 屬於return a new result container.
6,4合併返回6，屬於The combiner function may fold state from one argument into the other and return that。
*/
/**
* Perform the final transformation from the intermediate accumulation type
* {@code A} to the final result type {@code R}.
*R 是最終返回結果的類型。
* <p>If the characteristic {@code IDENTITY_TRANSFORM} is
* set, this function may be presumed to be an identity transform with an
* unchecked cast from {@code A} to {@code R}.
*
* @return a function which transforms the intermediate result to the final
* result
*/
Function<A, R> finisher(); // 合併中間的值，給出返回值。
/**
* Returns a {@code Set} of {@code Collector.Characteristics} indicating
* the characteristics of this Collector. This set should be immutable.
*
* @return an immutable set of collector characteristics
*/
Set<Characteristics> characteristics(); //特徵的集合
/**
* Returns a new {@code Collector} described by the given {@code supplier},
* {@code accumulator}, and {@code combiner} functions. The resulting
* {@code Collector} has the {@code Collector.Characteristics.IDENTITY_FINISH}
* characteristic.
*
* @param supplier The supplier function for the new collector
* @param accumulator The accumulator function for the new collector
* @param combiner The combiner function for the new collector
* @param characteristics The collector characteristics for the new
* collector
* @param <T> The type of input elements for the new collector
* @param <R> The type of intermediate accumulation result, and final result,
* for the new collector
* @throws NullPointerException if any argument is null
* @return the new {@code Collector}
*/
public static<T, R> Collector<T, R, R> of(Supplier<R> supplier,
BiConsumer<R, T> accumulator,
BinaryOperator<R> combiner,
Characteristics... characteristics) {
Objects.requireNonNull(supplier);
Objects.requireNonNull(accumulator);
Objects.requireNonNull(combiner);
Objects.requireNonNull(characteristics);
Set<Characteristics> cs = (characteristics.length == 0)
? Collectors.CH_ID
: Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH,
characteristics));
return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, cs);
}
/**
* Returns a new {@code Collector} described by the given {@code supplier},
* {@code accumulator}, {@code combiner}, and {@code finisher} functions.
*
* @param supplier The supplier function for the new collector
* @param accumulator The accumulator function for the new collector
* @param combiner The combiner function for the new collector
* @param finisher The finisher function for the new collector
* @param characteristics The collector characteristics for the new
* collector
* @param <T> The type of input elements for the new collector
* @param <A> The intermediate accumulation type of the new collector
* @param <R> The final result type of the new collector
* @throws NullPointerException if any argument is null
* @return the new {@code Collector}
*/
public static<T, A, R> Collector<T, A, R> of(Supplier<A> supplier,
BiConsumer<A, T> accumulator,
BinaryOperator<A> combiner,
Function<A, R> finisher,
Characteristics... characteristics) {
Objects.requireNonNull(supplier);
Objects.requireNonNull(accumulator);
Objects.requireNonNull(combiner);
Objects.requireNonNull(finisher);
Objects.requireNonNull(characteristics);
Set<Characteristics> cs = Collectors.CH_NOID;
if (characteristics.length > 0) {
cs = EnumSet.noneOf(Characteristics.class);
Collections.addAll(cs, characteristics);
cs = Collections.unmodifiableSet(cs);
}
return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, finisher, cs);
}
/**
* Characteristics indicating properties of a {@code Collector}, which can
* be used to optimize reduction implementations.
*/
enum Characteristics { // 特徵
/**
* Indicates that this collector is <em>concurrent</em>, meaning that
* the result container can support the accumulator function being
* called concurrently with the same result container from multiple
* threads.
*
* <p>If a {@code CONCURRENT} collector is not also {@code UNORDERED},
* then it should only be evaluated concurrently if applied to an
* unordered data source.
*/
CONCURRENT,
/**
* Indicates that the collection operation does not commit to preserving
* the encounter order of input elements. (This might be true if the
* result container has no intrinsic order, such as a {@link Set}.)
*/
UNORDERED,
/**
* Indicates that the finisher function is the identity function and
* can be elided. If set, it must be the case that an unchecked cast
* from A to R will succeed.
*/
IDENTITY_FINISH
}
}
本文是針對collect類源碼的詳解。必要的中文總結的感受挺好的。有錯誤的地方歡迎指出。嘿嘿。一塊兒慢慢成長哦。java
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