Java和Android中，代码块、static静态代码块的执行顺序

    <p>Java和Android中，代码块、static静态代码块的执行顺序有没有什么区别呢。</p>

Java

先来个简单的例子

Main.java:

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public class Main {
    static int a = 1;
    static {
        System.out.println(a);
    }
    static {
        a = 2;
    }
    public static void main(String[] args) {
        System.out.println("Hello World!");
        System.out.println(a);
    }
}

输出为

1
Hello World!
2

尝试交换static变量声明和static代码块后，编译报错”非法前向引用”。
交互两个代码块位置后输出

2
Hello World!
2

可见static修饰的，是从上向下，依次执行的。static变量和代码块优先执行

我们再来做一个更复杂的实验

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public class Main {
    static int a = 1;
    static {
        System.out.println(a);
    }
    static {
        a = 2;
    }
    public static void main(String[] args) {
        System.out.println("Hello World!");
        Yo.test();
        Yo yooo = new Yo();
        yooo.test();
    }
}
public class Yo {
    static String name = "mark";
    static {
        System.out.println(name);
    }
    public Yo() {
        super();
        System.out.println("constructor");
    }
    static public void test() {
        System.out.println("hello " + name);
    }
}

输出

1
Hello World!
mark
hello mark
constructor
hello mark

Yo的执行符合上面的结论，执行了static代码块和静态方法test，构造Yo实例后，没有重复执行static代码块。

Android

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public class TestApplication extends BaseApplication {
    static {
        Log.d("test", "Application 1123");
    }
    static int a = 1;
    static {
        Log.d("test", "Application " + a);
        HomeActivity.test();
    }
    static {
        a = 2;
    }
    @Override
    protected void attachBaseContext(Context base) {
        Log.d("test", "Application hello " + a);
    }
}
public class SplashActivity extends BaseActivity {
    static {
        Log.d("test", "SplashActivity static code block");
    }
    static int a = 1;
    static {
        Log.d("test", "SplashActivity " + a);
    }
    static {
        a = 2;
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Log.d("test", "SplashActivity onCreate " + a);
        // 省略，会去start HomeActivity
    }
}
public class HomeActivity extends BaseActivity {
    public static final String TAG = "HomeActivity";
    static {
        Log.d("test", "HomeActivity 1123");
    }
    public static void test() {
        Log.d("test", "HomeActivity");
    }
}

输出结果为

D/test﹕ Application 1123
D/test﹕ Application 1
D/test﹕ HomeActivity 1123
D/test﹕ HomeActivity
D/test﹕ Application hello 2
D/test﹕ SplashActivity static code block
D/test﹕ SplashActivity 1
D/test﹕ SplashActivity onCreate 2

从结果上看java并没有什么区别

同样是先执行依次static，入口外其他类的static初始化在类被JVM加载（从结果来看，是在类被使用的时候才会加载）的时候执行。

转自:http://blog.zhaiyifan.cn/2015/08/28/Java%E5%92%8CAndroid%E4%B8%AD%EF%BC%8C%E4%BB%A3%E7%A0%81%E5%9D%97%E3%80%81static%E9%9D%99%E6%80%81%E4%BB%A3%E7%A0%81%E5%9D%97%E7%9A%84%E6%89%A7%E8%A1%8C%E9%A1%BA%E5%BA%8F/