Android 注解的一些应用以及原理

String a=null;
sayHello(a);

Argument 'a' might be null less... (Ctrl+F1)
This inspection analyzes method control and data flow to report possible conditions that are always true or false, expressions whose value is statically proven to be constant, and situations that can lead to nullability contract violations.
Variables, method parameters and return values marked as @Nullable or @NotNull are treated as nullable (or not-null, respectively) and used during the analysis to check nullability contracts, e.g. report possible NullPointerException errors.
More complex contracts can be defined using @Contract annotation, for example:
@Contract("_, null -> null") — method returns null if its second argument is null @Contract("_, null -> null; _, !null -> !null") — method returns null if its second argument is null and not-null otherwise @Contract("true -> fail") — a typical assertFalse method which throws an exception if true is passed to it
The inspection can be configured to use custom @Nullable
@NotNull annotations (by default the ones from annotations.jar will be used)

private String sayHello(@StringRes int resId) {
return "";
}

sayHello(R.layout.activity_main);

class TestAnoataion { public void testAnoation() {
Log.v("main", "test anoation");
}
} class TestAnoataion2 extends TestAnoataion
{
public void testAnoation()
{ }
}

package com.example.administrator.testfab; import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target; /**
* Created by Administrator on 2015/8/5.
*/ @Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface InjectView {
//id就表示哪些控件，-1就表示取不到时候的默认值
int id() default -1;
}

package com.example.administrator.testfab; import android.app.Activity;
import android.view.View; import java.lang.reflect.Field; /**
* Created by Administrator on 2015/8/5.
*/
public class InjectViewParser { public static void inject(Object object) { try {
parse(object);
} catch (Exception e) {
e.printStackTrace();
}
} public static void parse(Object object) throws Exception {
final Class<?> clazz = object.getClass();
View view = null;
Field[] fields = clazz.getDeclaredFields();
for (Field field : fields) {
if (field.isAnnotationPresent(InjectView.class)) {
InjectView injectView = field.getAnnotation(InjectView.class);
int id = injectView.id();
if (id < 0) {
throw new Exception("id must not be null");
} else {
field.setAccessible(true);
if (object instanceof View) {
view = ((View) object).findViewById(id);
} else if (object instanceof Activity) {
view = ((Activity) object).findViewById(id);
}
field.set(object, view);
} } } }
}

package com.example.administrator.testfab; import android.os.AsyncTask;
import android.os.Bundle;
import android.support.annotation.CallSuper;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.annotation.StringRes;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button; public class MainActivity extends AppCompatActivity { @InjectView(id = R.id.bt)
private Button bt; @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//开始注入
InjectViewParser.inject(this);
//这个主要是测试注入id 成功没有 成功了就不会报错~
bt.setText("inject done"); } @Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
} @Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId(); //noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
} return super.onOptionsItemSelected(item);
} }