Java synchronized keyword details

In fact, more than 90% of my basic java knowledge comes from Thinking in Java. For the synchronized keyword, you just browsed it and probably didn't take it into detail. This keyword has never been used. Yesterday I saw the Thinking in Patterns with the Observer mode in Java. I checked the source code of the Observable class and found that almost all of the methods in it used the synchronized keyword (not all ), synchronized (this) {} blocks are used individually. So I found it necessary to understand this keyword.

I read the Thinking in Java second Chinese Version translated by Hou Jie again and got the following information about synchronized:

1. the synchronized keyword has two scopes:

1) In an object instance, synchronized aMethod () {} can prevent multiple threads from simultaneously accessing the synchronized Method of the object (if an object has multiple synchronized methods, as long as a thread accesses one of the synchronized methods, other threads cannot simultaneously access any of this object's synchronized methods ). At this time, the synchronized methods of different object instances are irrelevant. That is to say, other threads can access the synchronized Method in another object instance of the same class at the same time;

2) is the scope of a class. synchronized static aStaticMethod {} prevents multiple threads from simultaneously accessing the synchronized static method in this class. It can work on all object instances of the class.

2. In addition to using the synchronized keyword before the method, the synchronized keyword can also be used in a block in the method, indicating that only the resources in this block are mutually exclusive. Usage: synchronized (this) {/* block */}. Its scope is the current object;

3. the synchronized keyword cannot be inherited. That is to say, the method of the base class synchronized f () {} is not automatically synchronized f () {} in the inheritance class (){}, instead, it becomes f (){}. You need to explicitly specify a method of the inherited class as the synchronized method;

4. Questions:

I know that there is a static block (the loading time is also unclear, and I need to learn about it. It turns out Thinking in Java seems to be saying: when a static block is loaded, an object instance of the class is created or a static method of the class is accessed, but this morning I read a piece of information on javaeye.com about when to execute the <static block? -The article "eway-JavaEye technology community>" is a bit confused later. :) maybe, you need to carefully learn the internal mechanism of JVM, which is the most fundamental way to understand such problems ), there is also a synchronized block. Is there a synchronized static block? Is there a synchronized block in the class scope?