Do we need to synchronize writes if we are synchro

问题:

I have few doubts about synchronized blocks. Before my questions I would like to share the answers from another related post Link for Answer to related question. I quote Peter Lawrey from the same answer.

1. synchronized ensures you have a consistent view of the data. This means you will read the latest value and other caches will get the latest value. Caches are smart enough to talk to each other via a special bus (not something required by the JLS, but allowed) This bus means that it doesn't have to touch main memory to get a consistent view.

2. If you only use synchronized, you wouldn't need volatile. Volatile is useful if you have a very simple operation for which synchronized would be overkill.

In reference to above I have three questions below :

Q1. Suppose in a multi threaded application there is an object or a primitive instance field being only read in a synchronized block (write may be happening in some other method without synchronization). Also Synchronized block is defined upon some other Object. Does declaring it volatile (even if it is read inside Synchronized block only) makes any sense ?

Q2. I understand the value of the states of the object upon which Synchronization has been done is consistent. I am not sure for the state of other objects and primitive fields being read in side the Synchronized block. Suppose changes are made without obtaining a lock but reading is done by obtaining a lock. Does state of all the objects and value of all primitive fields inside a Synchronized block will have consistent view always. ?

Q3. [Update] : Will all fields being read in a synchronized block will be read from main memory regardless of what we lock on ? [answered by CKing]

I have a prepared a reference code for my questions above.

public class Test {
  private SomeClass someObj;
  private boolean isSomeFlag;
  private Object lock = new Object();
  public SomeClass getObject() {
        return someObj;
  }
  public void setObject(SomeClass someObj) {
        this.someObj = someObj;
  }
  public void executeSomeProcess(){
        //some process...
  }
  // synchronized block is on a private someObj lock.
  // inside the lock method does the value of isSomeFlag and state of someObj remain consistent?

  public void someMethod(){
        synchronized (lock) {
              while(isSomeFlag){
                    executeSomeProcess();
              }
              if(someObj.isLogicToBePerformed()){
                    someObj.performSomeLogic();
              }
        }
  }
  // this is method without synchronization.
  public void setSomeFlag(boolean isSomeFlag) {
        this.isSomeFlag = isSomeFlag;
  }
}

回答1:

The first thing you need to understand is that there is a subtle difference between the scenario being discussed in the linked answer and the scenario you are talking about. You speak about modifying a value without synchronization whereas all values are modified within a synchronized context in the linked answer. With this understanding in mind, let's address your questions :

Q1. Suppose in a multi threaded application there is an object or a primitive instance field being only read in a synchronized block (write may be happening in some other method without synchronization). Also Synchronized block is defined upon some other Object. Does declaring it volatile (even if it is read inside Synchronized block only) makes any sense ?

Yes it does make sense to declare the field as volatile. Since the write is not happening in a synchronized context, there is no guarantee that the writing thread will flush the newly updated value to main memory. The reading thread may still see inconsistent values because of this.

Suppose changes are made without obtaining a lock but reading is done by obtaining a lock. Does state of all the objects and value of all primitive fields inside a Synchronized block will have consistent view always. ?

The answer is still no. The reasoning is the same as above.

Bottom line : Modifying values outside synchronized context will not ensure that these values get flushed to main memory. (as the reader thread may enter the synchronized block before the writer thread does) Threads that read these values in a synchronized context may still end up reading older values even if they get these values from the main memory.

---

Note that this question talks about primitives so it is also important to understand that Java provides Out-of-thin-air safety for 32-bit primitives (all primitives except long and double) which means that you can be assured that you will atleast see a valid value (if not consistent).

回答2:

All synchronized does is capture the lock of the object that it is synchronized on. If the lock is already captured, it will wait for its release. It does not in any way assert that that object's internal fields won't change. For that, there is volatile

回答3:

When you synchronize on an object monitor A, it is guaranteed that another thread synchronizing on the same monitor A afterwards will see any changes made by the first thread to any object. That's the visibility guarantee provided by synchronized, nothing more.

A volatile variable guarantees visibility (for the variable only, a volatile HashMap doesn't mean the contents of the map would be visible) between threads regardless of any synchronized blocks.