英文很差，所以附上原文。
關係大概是這樣吧
Processes And Threads
Processes (進程)
每個 Process 都可視為一個獨立運作的環境，有自己完整、私有的 Run-Time 資源，特別是每個 process 都有自己的記憶體空間。
Processes 聽起來很像 programs 或 application 的另一種說法，然而事實上一個 application 有可能是許多 processes 合作組成的。為了讓 processes 彼此互相合作，一些作業系統支援了 Inter Process Communication (IPC) resources，例如 pipes 與 sockets。IPC 不只讓不同的 processes 互相通訊，同時也支援不同系統的 pocesses 互相通訊。
大部分 JVM 實現的都是 single process。Java Application 可以創建額外的 process 透過 ProcessBuilder Object。
A process has a self-contained execution environment. A process generally has a complete, private set of basic run-time resources; in particular, each process has its own memory space.
Processes are often seen as synonymous with programs or applications. However, what the user sees as a single application may in fact be a set of cooperating processes. To facilitate communication between processes, most operating systems support Inter Process Communication (IPC) resources, such as pipes and sockets. IPC is used not just for communication between processes on the same system, but processes on different systems.
Most implementations of the Java virtual machine run as a single process. A Java application can create additional processes using a ProcessBuilder object. Multiprocess applications are beyond the scope of this lesson.
Threads （線程）
Thread 在某些時候被視為一種輕量的 processes (lightweight processes)，與 processes 一樣，threads 也提供了一個可執行的環境。但相對於 processes 創建時，threads 並不需要太多的資源。
每個 process 最少都有一個 thread，threads 共享 processs 上的資源，包含 memory 與 files。這樣使得效率提高，但相對的也產生了一些潛在的問題，比如說 Race-condition 之類的問題。
Multithreaded 是 Java 平台上重要的一個特徵，每個 application 最少會有一個 thread。如果你去計算 "System" 上的 threads ，他們做的事情相似於記憶體管理或一些信號處理。但在這裡，我們只需要先認識 Main Thread，因為它能夠去創建額外的 threads。
Threads are sometimes called lightweight processes. Both processes and threads provide an execution environment, but creating a new thread requires fewer resources than creating a new process.
Threads exist within a process — every process has at least one. Threads share the process's resources, including memory and open files. This makes for efficient, but potentially problematic, communication.
Multithreaded execution is an essential feature of the Java platform. Every application has at least one thread — or several, if you count "system" threads that do things like memory management and signal handling. But from the application programmer's point of view, you start with just one thread, called the main thread. This thread has the ability to create additional threads, as we'll demonstrate in the next section.
使用時機
- IO bound task，避免 IO blocking 的阻塞。
- CPU bound task，避免長時間大運算的邏輯的阻塞，藉由 threads 的使用，有效率地利用 CPU 。
- Schedule，ScheduledThreadPoolExecutor 會用到 threads
- Daemon，daemon 是一種在 backgrouopd 運作的 thread，當所有的 non-daemon 都結束了，daemon 就會自動終止。另外從 daemon 裡面建立出來的 thread 也都會歸類在 daemon-thread。
Life cycle
圖片來自：https://github.com/JustinSDK/JavaSE6Tutorial/blob/master/docs/CH15.md#1514-執行緒生命周期
Synchonized 同步
public synchronized void myMethod() {
//code
}
synchonized 用來保證一次只會有一個 thread 對某個資源的存取權，多執行緒對相同的 Object 做存取時，會發生：
DeadLock
比較經典的例子，哲學家吃飯
https://www.tutorialspoint.com/java/java_thread_deadlock.htm
Race Condition
比較經典的例子，應該就是提款機、賣票。
A 帳戶同時間發生， user1 與 user2 匯款，如果沒有 synchonized 的處理，會有
- user1 與 user2 都匯款成功, save
- user1 與 user2 都匯款成功, user2 的匯款資料蓋過 user1 , 損失 user1 的匯款金額。
- user1 與 user2 都匯款成功, user1 的匯款資料蓋過 user2 , 損失 user2 的匯款金額。