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类加载的并发，单例模式，静态资源加载

全限定类在同一个类加载器只能加载一次，意味着static对象及代码块只一次，为单例之依据

如果并发发生，则阻塞

故类的加载不存在多线程，因为只执行一次，其他线程等着加载线程，由jvm来保证线程安全性

public class ByLoad {

    static {
        try {
            System.out.println(Thread.currentThread() + " start " + System.currentTimeMillis()/1000 );
            Thread.sleep(20000);　　【11行】
            System.out.println(Thread.currentThread() + " end " + System.currentTimeMillis()/1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void func() {
        System.out.println(Thread.currentThread() + " func " + System.currentTimeMillis()/1000);
    }
}

public class Main {
    public static void main(String []f) {
        CountDownLatch countDownLatch = new CountDownLatch(1);
        for(int i=0; i<3; ++i) {
            int id = i+1;
            Thread thread = new Thread(new MyThread(countDownLatch), "thread - " + id + " haha");
            thread.start();
        }
        countDownLatch.countDown();

    }

    private static class MyThread implements Runnable {

        private CountDownLatch countDownLatch;

        public MyThread(CountDownLatch countDownLatch) {
            this.countDownLatch = countDownLatch;
        }

        @Override
        public void run() {
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            ByLoad.func(); 【35行】
        }
    }
}

输出：

Thread[thread - 1 haha,5,main] start 1602256385
Thread[thread - 1 haha,5,main] end 1602256405
Thread[thread - 1 haha,5,main] func 1602256405
Thread[thread - 3 haha,5,main] func 1602256405
Thread[thread - 2 haha,5,main] func 1602256405

可以看到线程1执行到ByLoad.func()时，进行ByLoad的加载，静态代码块执行

2 3 线程的ByLoad.func()被阻塞

"thread - 3 haha" #12 prio=5 os_prio=31 tid=0x00007f820199a000 nid=0x4003 in Object.wait() [0x0000700002cf2000]
java.lang.Thread.State: RUNNABLE
at Thread.loader.Main$MyThread.run(Main.java:35)
at java.lang.Thread.run(Thread.java:745)

"thread - 2 haha" #11 prio=5 os_prio=31 tid=0x00007f820003d800 nid=0x4103 in Object.wait() [0x0000700002bef000]
java.lang.Thread.State: RUNNABLE
at Thread.loader.Main$MyThread.run(Main.java:35)
at java.lang.Thread.run(Thread.java:745)

"thread - 1 haha" #10 prio=5 os_prio=31 tid=0x00007f8201837800 nid=0x4203 waiting on condition [0x0000700002aeb000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Thread.loader.ByLoad.<clinit>(ByLoad.java:11)
at Thread.loader.Main$MyThread.run(Main.java:35)
at java.lang.Thread.run(Thread.java:745)

3 我们来回看单例模式

饿汉式

public class ImageLoader{  
     private static ImageLoader instance = new ImageLoader;  
     private ImageLoader(){}  
     public static ImageLoader getInstance(){   
          return instance;   
      }  
}

一上来就把单例对象创建出来了，要用的时候直接返回即可

这里的“一上来”是有误区的，java的类本来就是懒加载，不存在一上来；

那么为什么还会出来后面的静态代码块式单例模式？

试想一下这种情况：

public class ImageLoader{

private static ImageLoader instance = new ImageLoader;

private ImageLoader(){}

public static ImageLoader getInstance(){

return instance;

}

　　public static void any() {...}

}

any() 先于getInstance执行，则不算懒加载了

故：

为了避免意外提前加载，在实际业务代码中才引入静态内部类加固懒加载，而不完全指望公开类懒加载，

单例的静态内部类模式即是如此原理

6 我们常常碰到静态基础数据加载：

6.1 双检

public class CurrencyDataProvider implements DataProvider<String> {

    @Inject
    private SCEFCacheUtils scefCacheUtils;

    private static List<String> result;

    @Override
    public Collection<String> getAll() {
        if (result == null) {
            synchronized (CurrencyDataProvider.class) {
                if (result == null) {
                    result = new ArrayList<>(scefCacheUtils.getCurrencyList());
                    Collections.sort(result);
                }
            }
        }
        return result;
    }
}

比较好理解，然后sonar抵触double-check lock

6.2

public class CurrencyDataProvider implements DataProvider<String> {

    @Inject
    private SCEFCacheUtils scefCacheUtils;

    private static List<String> result;

    @Override
    public Collection<String> getAll() {
        if (result == null) {
            result = new ArrayList<>(scefCacheUtils.getCurrencyList());
            Collections.sort(result);
        }
        return result;
    }
}

那就拆锁吧，首次访问并发时无法多查几次库，毕竟是读操作

然后被sonar指出，sort（result）有并发访问的风险-大bug

6.3

public class CurrencyDataProvider implements DataProvider<String> {

    @Inject
    private SCEFCacheUtils scefCacheUtils;

    private static List<String> result;

    @Override
    public Collection<String> getAll() {
        if (result == null) {
            List<String> list = new ArrayList<>(scefCacheUtils.getCurrencyList());
            Collections.sort(list);
            result = list;
        }
        return result;
    }
}

好我们改一下，让局部对象参与sort，搞定了再给类对象，并发时，最多是多次对result赋值

然而sonar又不干了，认为static result被并发了，非线程安全

6.4

public class CurrencyDataProvider implements DataProvider<String> {

    @Override
    public Collection<String> getAll() {
        return InnerLoader.res;
    }

    private static class InnerLoader {
        private static List<String> res;

        static {
            Injector injector = CRFGuiceContext.getInjector();
            SCEFCacheUtils scefCacheUtils1 = injector.getInstance(SCEFCacheUtils.class);
            List<String> list = new ArrayList<>(scefCacheUtils1.getCurrencyList());
            Collections.sort(list);
            res = list;
        }
    }
}

静态内部类-sonar推荐的方案

6.4.1 使用static代码块进行static资源加载，因为第1点指出static代码块和对象只会搞一次，这样就避免了并发重复加载，且替代了double-check

6.4.2 为啥不直接写在CurrencyDataProvider的static代码块？

因为第4点指出，如果CurrencyDataProvider被意外loadclass或forname或其他方式率先加载了，执行static代码块时，Guice环境可能还没好

6.5

public class CurrencyDataProvider implements DataProvider<String> {

    @Override
    public Collection<String> getAll() {
        return InnerLoader.res;
    }

    private static class InnerLoader {
        private static List<String> res;

        static {
            Injector injector = CRFGuiceContext.getInjector();
            SCEFCacheUtils scefCacheUtils1 = injector.getInstance(SCEFCacheUtils.class);
            res = new ArrayList<>(scefCacheUtils1.getCurrencyList());
            Collections.sort(res);
        }
    }
}

有了第1点的结论：类的加载不存在多线程，因为只执行一次，其他线程等着加载线程，我们放心的对static对象res直接进行sort，而不用局部对象缓冲，因为不会有第2个线程再执行到这儿了，只此一次执行

mybatis guice 事务代理切面中sqlSessionManager也可用：

//    private static SqlSessionManager sqlSessionManager = null;

    private SqlSessionManager getOdsSqlSessionManager() {
        return SqlManagerLoader.sqlSessionManager;
    }

    private static class SqlManagerLoader {
        private static SqlSessionManager sqlSessionManager = null;
        static {
            OdsTransactionMapper odsTransactionMapper = getBeanFromFactoryGuice(OdsTransactionMapper.class);
            InvocationHandler lvInvHandler0 = Proxy.getInvocationHandler(odsTransactionMapper);
            ManagedMapperProvider managedMapperProvider0 = (ManagedMapperProvider) lvInvHandler0;
            sqlSessionManager = managedMapperProvider0.getSqlSessionManager();
        }
    }

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原文地址：https://www.cnblogs.com/silyvin/p/13789575.html