Java Apache HttClient：ThreadSafe吗？

Question

I have to create a web service which interrogate a black box via an http request (method get).

I would like to use Apache http client, but with 4.3.1 version i have some warning about deprecated object & methods.

All example explain to use DefaultHttpClient which is ThreadSafe. Now we have to use HttClientBuilder which is NOT ThreadSafe.

What is impact about that exactly ? Stupid question : if i use it in a Spring MVC Controller (Singleton threadsafe), what are the consequences ? Is a not thread safe process used in a thread safe become thread safe (lol) ?

Answer 1

下面的代码返回一个CloseableHttpClient ，如果您认为它是线程安全的。

HttpClientBuilder.create().build();

Answer 2

I don't know exactly, what you want to do, but concurrency is all about sharing state between threads. If two threads access the same state in parallel, you might/will get problems.

Note that local variables are NOT shared, so if multiple threads invoke the same method in parallel, all of them have their own local variables:

public int safeAdd(int a, int b) {
    int sum = a + b;
    return sum;
}

The local variables a , b and sum are NOT shared state! Every thread gets its own 'copy' of them! So the object is stateless!

Stateless objects are always thread-safe!

Of course, things change if you publish you local variables:

int sum;

public int unsafeAdd(int a, int b) {
    sum = a + b;
    return sum;
}

Here, sum is not a local variable any more. It exists only once (per instance). If multiple threads call unsafeAdd , something bad/unexpected might/will happen!

This means, if you use your HttpClientBuilder only within your methods - no problem. But if you publish it (as in the example above), you must take care of parallel access.

(See also book "Java Concurrency in Practice": http://jcip.net/ )

Answer 3

Seems you need to understand what thread-safe means first.

Class X being thread safe generally means that if you have a class variable Y (of type X) and you use it from multiple threads, this will not lead to any problems (eg because X's public methods are guarded/synchronized).

Saying for a thread/process that it's thread safe or not - that doesn't make much sense, it is about classes and objects of these classes.

When a class is not thread safe you can use objects of this class as local variables and not worry about thread-safety too.

That's because a local variable is not shared across threads, each thread has its own copy of it.

But if you use it as class variable you can have thread safety problems.