Java等GC语言中未关闭的资源会被GC自动关闭吗？以FileInputStream为例

Great questions—let's break these down with practical context, since this is a super common pitfall for folks new to GC languages:

Short answer: No, not reliably—and you should never count on it.

Here's why: Garbage collectors exist only to manage memory. Their sole job is to track objects in the heap that are no longer referenced, then free up the memory those objects take. They have zero awareness of "external resources" like file handles, network sockets, database connections, or hardware locks—these are managed by the operating system, not the JVM (or whatever runtime your GC language uses).

For example: If you open a network socket in Python and forget to close it, the GC will eventually collect the socket object and free its memory... but it won't tell the OS to release the socket connection. That connection could hang around until your program exits, causing resource leaks or even hitting OS-level limits (like max open sockets).

Some languages have optional cleanup hooks (like Java's old finalize()), but these are not part of the GC's core responsibility—they're afterthoughts, and never guaranteed to run when you need them.

Again, technically? Sometimes—but you should never rely on this behavior.

Back in older Java versions (pre-Java 9), FileInputStream (and similar IO classes) had a finalize() method that would call close() when the object was garbage collected. But here's the problem with that:

• Finalizers run at the GC's mercy: You have no idea when (or if) the GC will get around to collecting the FileInputStream object. Your program could keep running for hours, holding onto that file handle the whole time—eventually hitting the OS's limit for open files and crashing.
• Finalizers might never run: If your program exits before the GC collects the object, the finalizer won't execute at all, leaving the file handle open until the OS cleans it up (which might not happen right away).
• Finalizers are deprecated: Java 9 marked finalize() as deprecated, and it's being phased out entirely. Modern Java uses Cleaner or PhantomReference for resource cleanup, but these are still meant as safety nets—not primary cleanup mechanisms.

The only safe way to handle resources like FileInputStream is to close them explicitly. In Java, the best practice is to use try-with-resources syntax, which guarantees the resource is closed even if an exception is thrown:

try (FileInputStream fis = new FileInputStream("myfile.txt")) {
    // Read from the file here
} catch (IOException e) {
    // Handle any errors
}

This way, you're in control of resource cleanup, not the unpredictable GC.

内容的提问来源于stack exchange，提问作者wozuiqiangdeaoyi