Why Your `.parallelStream()` Might Not Be Parallel at All

When we use .parallelStream() in Java, we expect parallelism to magically kick in — blazing fast execution across multiple threads.

But sometimes, even with .parallelStream(), everything still runs in a single thread. No speedup. No parallelism. Just disappointment.

🧪 The Initial Code

I had this pattern:

<span>List</span><span><</span><span>String</span><span>></span> <span>users</span> <span>=</span> <span>List</span><span>.</span><span>of</span><span>(</span><span>"alice"</span><span>,</span> <span>"bob"</span><span>);</span>
<span>List</span><span><</span><span>String</span><span>></span> <span>products</span> <span>=</span> <span>IntStream</span><span>.</span><span>range</span><span>(</span><span>0</span><span>,</span> <span>10_000</span><span>)</span>
    <span>.</span><span>mapToObj</span><span>(</span><span>i</span> <span>-></span> <span>"product-"</span> <span>+</span> <span>i</span><span>)</span>
    <span>.</span><span>toList</span><span>();</span>
<span>List</span><span><</span><span>String</span><span>></span> <span>result</span> <span>=</span> <span>users</span><span>.</span><span>stream</span><span>()</span>
    <span>.</span><span>flatMap</span><span>(</span><span>user</span> <span>-></span>
        <span>products</span><span>.</span><span>parallelStream</span><span>()</span>
            <span>.</span><span>map</span><span>(</span><span>product</span> <span>-></span> <span>{</span>
                <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>"Sequential map: "</span> <span>+</span> <span>user</span> <span>+</span> <span>" - "</span> <span>+</span> <span>product</span> <span>+</span> <span>" on "</span> <span>+</span> <span>Thread</span><span>.</span><span>currentThread</span><span>().</span><span>getName</span><span>());</span>
                <span>return</span> <span>user</span> <span>+</span> <span>" → "</span> <span>+</span> <span>product</span><span>;</span>
            <span>})</span>
    <span>)</span>
    <span>.</span><span>toList</span><span>();</span>
<span>List</span><span><</span><span>String</span><span>></span> <span>users</span> <span>=</span> <span>List</span><span>.</span><span>of</span><span>(</span><span>"alice"</span><span>,</span> <span>"bob"</span><span>);</span>
<span>List</span><span><</span><span>String</span><span>></span> <span>products</span> <span>=</span> <span>IntStream</span><span>.</span><span>range</span><span>(</span><span>0</span><span>,</span> <span>10_000</span><span>)</span>
    <span>.</span><span>mapToObj</span><span>(</span><span>i</span> <span>-></span> <span>"product-"</span> <span>+</span> <span>i</span><span>)</span>
    <span>.</span><span>toList</span><span>();</span>

<span>List</span><span><</span><span>String</span><span>></span> <span>result</span> <span>=</span> <span>users</span><span>.</span><span>stream</span><span>()</span>
    <span>.</span><span>flatMap</span><span>(</span><span>user</span> <span>-></span>
        <span>products</span><span>.</span><span>parallelStream</span><span>()</span>
            <span>.</span><span>map</span><span>(</span><span>product</span> <span>-></span> <span>{</span>
                <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>"Sequential map: "</span> <span>+</span> <span>user</span> <span>+</span> <span>" - "</span> <span>+</span> <span>product</span> <span>+</span> <span>" on "</span> <span>+</span> <span>Thread</span><span>.</span><span>currentThread</span><span>().</span><span>getName</span><span>());</span>
                <span>return</span> <span>user</span> <span>+</span> <span>" → "</span> <span>+</span> <span>product</span><span>;</span>
            <span>})</span>
    <span>)</span>
    <span>.</span><span>toList</span><span>();</span>
List<String> users = List.of("alice", "bob");
List<String> products = IntStream.range(0, 10_000)
    .mapToObj(i -> "product-" + i)
    .toList();

List<String> result = users.stream()
    .flatMap(user ->
        products.parallelStream()
            .map(product -> {
                System.out.println("Sequential map: " + user + " - " + product + " on " + Thread.currentThread().getName());
                return user + " → " + product;
            })
    )
    .toList();

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Looks fine, right?

🤯 But It Wasn’t Parallel

When I ran it, the output showed every single operation happening in the same thread.

Even though I used .parallelStream()!

Turns out this pattern is broken. Why?

🧠 Why It Doesn’t Work

A much better and simpler explanation than my original article was shared by aleatorio.dev.br on Bluesky

“The answer lies in the implementation of flatMap, which takes a parallel stream and forcibly calls .sequential() on it, turning it back into a sequential stream.”

So even if you write:

<span>products</span><span>.</span><span>parallelStream</span><span>().</span><span>map</span><span>(...)</span>
<span>products</span><span>.</span><span>parallelStream</span><span>().</span><span>map</span><span>(...)</span>
products.parallelStream().map(...)

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If that stream is returned inside a flatMap(), Java will implicitly make it sequential again — effectively canceling your parallelism.

I highly recommend checking out aleatorio.dev.br’s full post (I’ll link it in the comments).

The Fix: Flip the Streams

Instead of looping over users and creating parallel streams of products, loop over the products:

<span>List</span><span><</span><span>String</span><span>></span> <span>result</span> <span>=</span> <span>products</span><span>.</span><span>parallelStream</span><span>()</span>
    <span>.</span><span>flatMap</span><span>(</span><span>product</span> <span>-></span>
        <span>users</span><span>.</span><span>stream</span><span>()</span>
            <span>.</span><span>map</span><span>(</span><span>user</span> <span>-></span> <span>{</span>
                <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>"Parallel map: "</span> <span>+</span> <span>user</span> <span>+</span> <span>" - "</span> <span>+</span> <span>product</span> <span>+</span> <span>" on "</span> <span>+</span> <span>Thread</span><span>.</span><span>currentThread</span><span>().</span><span>getName</span><span>());</span>
                <span>return</span> <span>user</span> <span>+</span> <span>" → "</span> <span>+</span> <span>product</span><span>;</span>
            <span>})</span>
    <span>)</span>
    <span>.</span><span>toList</span><span>();</span>
<span>List</span><span><</span><span>String</span><span>></span> <span>result</span> <span>=</span> <span>products</span><span>.</span><span>parallelStream</span><span>()</span>
    <span>.</span><span>flatMap</span><span>(</span><span>product</span> <span>-></span>
        <span>users</span><span>.</span><span>stream</span><span>()</span>
            <span>.</span><span>map</span><span>(</span><span>user</span> <span>-></span> <span>{</span>
                <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>"Parallel map: "</span> <span>+</span> <span>user</span> <span>+</span> <span>" - "</span> <span>+</span> <span>product</span> <span>+</span> <span>" on "</span> <span>+</span> <span>Thread</span><span>.</span><span>currentThread</span><span>().</span><span>getName</span><span>());</span>
                <span>return</span> <span>user</span> <span>+</span> <span>" → "</span> <span>+</span> <span>product</span><span>;</span>
            <span>})</span>
    <span>)</span>
    <span>.</span><span>toList</span><span>();</span>
List<String> result = products.parallelStream()
    .flatMap(product ->
        users.stream()
            .map(user -> {
                System.out.println("Parallel map: " + user + " - " + product + " on " + Thread.currentThread().getName());
                return user + " → " + product;
            })
    )
    .toList();

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Now the outer stream is parallel — and that’s what matters.

Result

After flipping the stream:

• Threads used: yes
• CPU utilization: yes
• Job speed: improved
• Logs: showed lots of worker threads doing the job in parallel

It just took one line change to go from single-threaded sadness to full multicore glory. In my particular case, my process time went down from 45 min to 10 min!

TL;DR

• .parallelStream() only works if the stream itself is parallel
• Putting .parallelStream() inside a .flatMap() from a sequential stream won’t help
• Flip the loop: parallelize the outer stream, not the inner one
• Print Thread.currentThread().getName() to debug what’s really happening

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原文链接：Why Your `.parallelStream()` Might Not Be Parallel at All