【Lombok】幾分鐘搞懂Lomobok_02_註解說明

Lombok註解說明

@val：用在局部變量前面，相當於將變量聲明爲final
@NonNull：給方法參數增加這個註解會自動在方法內對該參數進行是否爲空的校驗，如果爲空，則拋出NPE（NullPointerException）
@Cleanup：自動管理資源，用在局部變量之前，在當前變量範圍內即將執行完畢退出之前會自動清理資源，自動生成try-finally這樣的代碼來關閉
@Getter/@Setter：用在屬性上，再也不用自己手寫setter和getter方法了，還可以指定訪問範
@ToString：用在類上，可以自動覆寫toString方法，當然還可以加其他參數，例如@ToString(exclude=”id”)排除id屬性，或者@ToString(callSuper=true, includeFieldNames=true)調用父類的toString方法，包含所有屬性
@EqualsAndHashCode：用在類上，自動生成equals方法和hashCode方法
@NoArgsConstructor, @RequiredArgsConstructor and @AllArgsConstructor：用在類上，自動生成無參構造和使用所有參數的構造函數以及把所有@NonNull屬性作爲參數的構造函數，如果指定staticName = “of”參數，同時還會生成一個返回類對象的靜態工廠方法，比使用構造函數方便很多
@Data：註解在類上，相當於同時使用了@ToString、@EqualsAndHashCode、@Getter、@Setter、@RequiredArgsConstrutor這些註解，對於POJO類十分有用
@Value：用在類上，是@Data的不可變形式，相當於爲屬性添加final聲明，只提供getter方法，而不提供setter方法
@Builder：用在類、構造器、方法上，爲你提供複雜的builder APIs，讓你可以像如下方式一樣調用Person.builder().name(“Adam Savage”).city(“San Francisco”).job(“Mythbusters”).job(“Unchained Reaction”).build();更多說明參考Builder
@SneakyThrows：自動拋受檢異常，而無需顯式在方法上使用throws語句
@Synchronized：用在方法上，將方法聲明爲同步的，並自動加鎖，而鎖對象是一個私有的屬性 $lock或$ LOCK，而java中的synchronized關鍵字鎖對象是this，鎖在this或者自己的類對象上存在副作用，就是你不能阻止非受控代碼去鎖this或者類對象，這可能會導致競爭條件或者其它線程錯誤
@Getter(lazy=true)：可以替代經典的Double Check Lock樣板代碼
@Log：根據不同的註解生成不同類型的log對象，但是實例名稱都是log，有六種可選實現類：

@CommonsLog Creates log = org.apache.commons.logging.LogFactory.getLog(LogExample.class);
@Log Creates log = java.util.logging.Logger.getLogger(LogExample.class.getName());
@Log4j Creates log = org.apache.log4j.Logger.getLogger(LogExample.class);
@Log4j2 Creates log = org.apache.logging.log4j.LogManager.getLogger(LogExample.class);
@Slf4j Creates log = org.slf4j.LoggerFactory.getLogger(LogExample.class);
@XSlf4j Creates log = org.slf4j.ext.XLoggerFactory.getXLogger(LogExample.class);

Lombok代碼示例

@val示例

public static void main(String[] args) {
    val sets = new HashSet<String>();
    val lists = new ArrayList<String>();
    val maps = new HashMap<String, String>();
    //=>相當於如下
    final Set<String> sets2 = new HashSet<>();
    final List<String> lists2 = new ArrayList<>();
    final Map<String, String> maps2 = new HashMap<>();
}

@NonNull示例

public void notNullExample(@NonNull String string) {
    string.length();
}
//=>相當於
public void notNullExample(String string) {
    if (string != null) {
        string.length();
    } else {
        throw new NullPointerException("null");
    }
}

@Cleanup示例

public static void main(String[] args) {
    try {
        @Cleanup InputStream inputStream = new FileInputStream(args[0]);
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    }
    //=>相當於
    InputStream inputStream = null;
    try {
        inputStream = new FileInputStream(args[0]);
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } finally {
        if (inputStream != null) {
            try {
                inputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

@Getter/@Setter示例

@Setter(AccessLevel.PUBLIC)
@Getter(AccessLevel.PROTECTED)
private int id;
private String shap;

@ToString示例

@ToString(exclude = "id", callSuper = true, includeFieldNames = true)
public class LombokDemo {
    private int id;
    private String name;
    private int age;
    public static void main(String[] args) {
        //輸出LombokDemo(super=LombokDemo@48524010, name=null, age=0)
        System.out.println(new LombokDemo());
    }
}

@EqualsAndHashCode示例

@EqualsAndHashCode(exclude = {"id", "shape"}, callSuper = false)
public class LombokDemo {
    private int id;
    private String shap;
}

@NoArgsConstructor, @RequiredArgsConstructor and @AllArgsConstructor示例

@NoArgsConstructor
@RequiredArgsConstructor(staticName = "of")
@AllArgsConstructor
public class LombokDemo {
    @NonNull
    private int id;
    @NonNull
    private String shap;
    private int age;
    public static void main(String[] args) {
        new LombokDemo(1, "circle");
        //使用靜態工廠方法
        LombokDemo.of(2, "circle");
        //無參構造
        new LombokDemo();
        //包含所有參數
        new LombokDemo(1, "circle", 2);
    }
}

@Data示例

import lombok.Data;
@Data
public class Menu {
    private String shopId;
    private String skuMenuId;
    private String skuName;
    private String normalizeSkuName;
    private String dishMenuId;
    private String dishName;
    private String dishNum;
    //默認閾值
    private float thresHold = 0;
    //新閾值
    private float newThresHold = 0;
    //總得分
    private float totalScore = 0;
}

@Value示例

@Value
public class LombokDemo {
    @NonNull
    private int id;
    @NonNull
    private String shap;
    private int age;
    //相當於
    private final int id;
    public int getId() {
        return this.id;
    }
    ...
}

@Builder示例

@Builder
public class BuilderExample {
    private String name;
    private int age;
    @Singular
    private Set<String> occupations;
    public static void main(String[] args) {
        BuilderExample test = BuilderExample.builder().age(11).name("test").build();
    }
}

@SneakyThrows示例

import lombok.SneakyThrows;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;
public class Test {
    @SneakyThrows()
    public void read() {
        InputStream inputStream = new FileInputStream("");
    }
    @SneakyThrows
    public void write() {
        throw new UnsupportedEncodingException();
    }
    //相當於
    public void read() throws FileNotFoundException {
        InputStream inputStream = new FileInputStream("");
    }
    public void write() throws UnsupportedEncodingException {
        throw new UnsupportedEncodingException();
    }
}

@Synchronized示例

public class SynchronizedDemo {
    @Synchronized
    public static void hello() {
        System.out.println("world");
    }
    //相當於
    private static final Object $LOCK = new Object[0];
    public static void hello() {
        synchronized ($LOCK) {
            System.out.println("world");
        }
    }
}

@Getter(lazy = true）

public class GetterLazyExample {
    @Getter(lazy = true)
    private final double[] cached = expensive();
    private double[] expensive() {
        double[] result = new double[1000000];
        for (int i = 0; i < result.length; i++) {
            result[i] = Math.asin(i);
        }
        return result;
    }
}

// 相當於如下所示: 
import java.util.concurrent.atomic.AtomicReference;
public class GetterLazyExample {
    private final AtomicReference<java.lang.Object> cached = new AtomicReference<>();
    public double[] getCached() {
        java.lang.Object value = this.cached.get();
        if (value == null) {
            synchronized (this.cached) {
                value = this.cached.get();
                if (value == null) {
                    final double[] actualValue = expensive();
                    value = actualValue == null ? this.cached : actualValue;
                    this.cached.set(value);
                }
            }
        }
        return (double[]) (value == this.cached ? null : value);
    }
    private double[] expensive() {
        double[] result = new double[1000000];
        for (int i = 0; i < result.length; i++) {
            result[i] = Math.asin(i);
        }
        return result;
    }
}

【Lombok】幾分鐘搞懂Lomobok_02_註解說明

【Lombok】幾分鐘搞懂Lomobok_02_註解說明

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【Lombok】幾分鐘搞懂Lomobok_02_註解說明

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