Java和Android中，代码块、static静态代码块的执行顺序

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。

Java

先来个简单的例子

Main.java:

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

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

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。

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