高并发下减少锁竞争

1.减少锁的持有时间，将不需要锁的操作从同步代码块的移除。

//可以优化的代码
class AttributeStore{
    private final Map<String,String> attributes=new HashMap<String,String>();
    public synchronized boolean userLocationMatches(String username,String regex){
        String key="user."+username;
        String location=attributes.get(key);
        if(location==null)
            return false;
        else
            return Pattern.matches(regex,location);
    }
}

//优化之后的代码
class AttributeStore{
    private final Map<String,String> attributes=new HashMap<String,String>();
    public boolean userLocationMatches(String username,String regex){
        String key="user."+username;
        String location;
        synchronized (this) {
            location=attributes.get(key);
        }
        if(location==null)
            return false;
        else
            return Pattern.matches(regex,location);
    }
}

2.降低锁的粒度

//可以锁分解的代码
class ServerStatus{
    private  Set<String> users;
    private  Set<String> queries;
    public synchronized void addUser(String user){
        users.add(user);
    }
    public synchronized void removeUser(String user){
        users.remove(user);
    }

    public synchronized void addQuery(String query){
        queries.add(query);
    }
    public synchronized void removeQuery(String query){
        queries.remove(query);
    }
}

//优化后的代码
class ServerStatus{
    private  Set<String> users;
    private  Set<String> queries;
    public  void addUser(String user){
        synchronized (users) {
            users.add(user);
        }
    }
    public  void removeUser(String user){
        synchronized (users) {
            users.remove(user);
        }
    }

    public  void addQuery(String query){
        synchronized (queries) {
            queries.add(query);
        }
    }
    public  void removeQuery(String query){
        synchronized (queries) {
            queries.remove(query);
        }
    }
}