今天看到KafkaProperties.java中对参数的设置使用了PropertyMapper,很好奇.就来看一看PropertyMapper的源码.
备注: 大多数说明都写在了注释里.

• 首先通过PropertyMapper.get()方法获取PropertyMapper中的静态实例变量(两个构造参数都为null).

private static final PropertyMapper INSTANCE = new PropertyMapper((PropertyMapper)null, (PropertyMapper.SourceOperator)null);
public static PropertyMapper get() {
    return INSTANCE;
}

• 然后使用alwaysApplyingWhenNonNull()方法创建一个新的PropertyMapper(以INSTANCE为parent,whenNonNull方法为SourceOperator),SourceOperator的作用是在from生成Source时追加处理返回的Source,后面会讲到.

public PropertyMapper alwaysApplyingWhenNonNull() {
    //把whenNonNull方法当成SourceOperator并作为参数调用alwaysApplying(SourceOperator operator)方法
    return alwaysApplying(this::whenNonNull);
}
// 对source执行whenNonNull()方法并返回,可用来当做SourceOperator接口作为上面方法的参数
private <T> Source<T> whenNonNull(Source<T> source) {
    return source.whenNonNull();
}

// 返回一个新的PropertyMapper实例,使用了给定 SourceOperator
public PropertyMapper alwaysApplying(SourceOperator operator) {
    Assert.notNull(operator, "Operator must not be null");
    return new PropertyMapper(this, operator);
}

public static final class Source<T> {
    private final Supplier<T> supplier;
    private final Predicate<T> predicate;

    private Source(Supplier<T> supplier, Predicate<T> predicate) {
        Assert.notNull(predicate, "Predicate must not be null");
        this.supplier = supplier;
        this.predicate = predicate;
    }
    // Source的whenNonNull方法返回一个新生成的Source,
    //对原来的supplier包装一个不会抛出空指针异常的supplier(避免source中的supplier为Null),
    //然后predicate(to时使用)则使用Object::nonNull.
    public Source<T> whenNonNull() {
        return new Source<>(new NullPointerExceptionSafeSupplier<>(this.supplier),Objects::nonNull);
    }

    //......略
}

//对传入的supplier封装一层,调用get时如果有空指针异常则进行捕获并return null
private static class NullPointerExceptionSafeSupplier<T> implements Supplier<T> {
    private final Supplier<T> supplier;
    NullPointerExceptionSafeSupplier(Supplier<T> supplier) {
        this.supplier = supplier;
    }
    @Override
    public T get() {
        try {
                return this.supplier.get();
        }catch (NullPointerException ex) {
                return null;
        }
    }
}

• 接着调用from方法构建PropertyMapper.Source :通过from方法传入一个Supplier接口,如果PropertyMapper没有parent(此处应该用了装饰者),则直接new出一个带CachingSupplier的Source.我们通过INSTANCE生成的PropertyMapper实例是以INSTANCE为实例的,故此处调用的是INSTANCE的from方法(其实最终还是走到了CachingSupplier)

private static final Predicate<?> ALWAYS = (t) -> { 
    return true;
};

/**
* 先对supplier封装一个CachingSupplier (见getSource方法)
* 然后又用sourceOperator封装一个NullPointerExceptionSafeSupplier(调用alwaysApplyingWhenNonNull方法返回的PropertyMapper进行此操作).
* 此处返回的Source对supplier进行了两层封装.
*/
public <T> PropertyMapper.Source<T> from(Supplier<T> supplier) {
    Assert.notNull(supplier, "Supplier must not be null");
    //getSource方法见下,包装一层带缓存的CachingSupplier
    PropertyMapper.Source<T> source = this.getSource(supplier);
   
    if (this.sourceOperator != null) {
        //此处使用了sourceOperator,默认情况下就是上文传入的source.whenNonNull方法.
        //故此处新返回source里的supplier是NullPointerExceptionSafeSupplier
        source = this.sourceOperator.apply(source);
    }
    //默认情况下,return的Source中的Supplier先后包装了CachingSupplier和NullPointerExceptionSafeSupplier
    return source;
}
private <T> PropertyMapper.Source<T> getSource(Supplier<T> supplier) {
    return this.parent != null ? 
        //如果存在parent则使用parent的from方法.此处使用了INSTANCE的from方法.但是最终还是走到了下面的CachingSupplier
        this.parent.from(supplier) : 
        //对传入的Supplier包装了一层带缓存(第一次调用会缓存)的Supplier, 
        //第二个是校验方法,使用的是默认的静态变量(无论如何都返回true,即不校验)
        new PropertyMapper.Source(new PropertyMapper.CachingSupplier(supplier), ALWAYS);
}

• 倒数两步是Source的as方法对结果的修改操作和to方法返回结果. 这两步就比较简单了,调用Function对结果进行修改(包装了层新的Supplier返回了新的Source)然后to调用Source的Supplier(supplier层层调用)返回结果.

public <R> Source<R> as(Function<T, R> adapter) {
    Assert.notNull(adapter, "Adapter must not be null");
    Supplier<Boolean> test = () -> this.predicate.test(this.supplier.get());
    Predicate<R> predicate = (t) -> test.get();
    Supplier<R> supplier = () -> {
        if (test.get()) {
            return adapter.apply(this.supplier.get());
        }
        return null;
    };
    //
    return new Source<>(supplier, predicate);
}
public void to(Consumer<T> consumer) {
    Assert.notNull(consumer, "Consumer must not be null");
    //这个supplier层层封装.
    T value = this.supplier.get();
    //符合predicate就执行consumer
    if (this.predicate.test(value)) {
        consumer.accept(value);
    }
}

• 测试

private static void sourceTest() {
        StringReturnTest origin = new StringReturnTest(), lastResult = new StringReturnTest();
        origin.setMsg("first set");
        PropertyMapper.Source<String> source =  PropertyMapper.get().alwaysApplyingWhenNonNull().from(origin::getMsg);
        origin.setMsg("second set");
        //执行to的时候才真正调用origin::getmsg
        source.as(s -> s).to(lastResult::setMsg);
        //输出 "second set"
        log.info("result:{}", lastResult.getMsg());
        //由于CacheSupplier的关系,此处返回依旧不变仍是"second set"
        origin.setMsg("third set");
        source.to(lastResult::setMsg);
        //输出 "second set"
        log.info("result:{}", lastResult.getMsg());
}

• 未经as的source的supplier可以debug时看到.