Java线程生命周期：Sun为何未定义Running状态？运行时状态是什么？

Great question—this gets to the core of how Java abstracts away platform-specific details to uphold its "write once, run anywhere" promise. Let's break this down clearly.

1. The Reason for Omitting the "Running" State #

Sun's call to merge "ready to run" and "actively executing" into a single Runnable state all comes down to abstraction and cross-platform consistency:

• Operating systems handle low-level thread scheduling differently across Windows, Linux, macOS, and other environments—things like CPU time slice allocation, context switching logic, and run queue management aren't standardized.
• From the JVM's perspective, it doesn't need to track whether a thread is currently using a CPU core or waiting in the OS's execution queue. Both scenarios mean the thread is unblocked and eligible to run. Adding a separate "Running" state would force the JVM to tie itself to OS-specific scheduling mechanisms, breaking Java's portability.
• The JVM only tracks states it can control: newly created threads, threads that are ready/executing, threads stuck waiting for resources, and threads that have finished execution. The line between "waiting for CPU" and "using CPU" is managed entirely by the OS, so the JVM abstracts that nuance away into one unified state.

2. Which State Corresponds to a Running Thread? #

When a thread is actively executing its code (i.e., the OS has assigned it a CPU time slice), it still falls under the Runnable state in the JVM's lifecycle definition.

To clarify the typical flow:

• After calling thread.start(), the thread moves from New to Runnable.
• While in Runnable, the thread might be waiting for the OS to schedule it, or it might be actively running—from the JVM's viewpoint, there's no meaningful distinction between these two scenarios.
• The thread only exits Runnable if it enters a blocked state (e.g., waiting for a synchronized lock, calling Thread.sleep(), or waiting for I/O) or terminates.

Quick test: If you call Thread.getState() on a thread that's currently executing code, it will return RUNNABLE, not a separate "Running" state.

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