Since you are already familiar with annotations now, the syntax should be very familiar.

public class Address {
    private String line1;
    private String line2;
    private String zip;
    private String state;
    private String country;
    private long id;
    
    // a not null string of 20 characters maximum
    @Length(max=20) 
    @NotNull
    public String getCountry() {
        return country;
    }
    
    // a non null string
    @NotNull
    public String getLine1() {
        return line1;
    }

    //no constraint    
    public String getLine2() {
        return line2;
    }
    
    // a not null string of 3 characters maximum
    @Length(max=3) @NotNull
    public String getState() {
        return state;
    }

    // a not null numeric string of 5 characters maximum
    // if the string is longer, the message will 
    //be searched in the resource bundle at key 'long'
    @Length(max=5, message="{long}")
    @Pattern(regex="[0-9]+")
    @NotNull
    public String getZip() {
        return zip;
    }
    
    // should always be true
    @AssertTrue
    public boolean isValid() {
        return true;
    }

    // a numeric between 1 and 2000
    @Id @Min(1)
    @Range(max=2000)
    public long getId() {
        return id;
    }
}

While the example only shows public property validation, you can also annotate fields of any kind of visibility.

@MyBeanConstraint(max=45)
public class Dog {
    @AssertTrue private boolean isMale;
    @NotNull protected String getName() { ... };
    ...
}

You can also annotate interfaces. Hibernate Validator will check all superclasses and interfaces extended or implemented by a given bean to read the appropriate validator annotations.

public interface Named {
    @NotNull String getName();
    ...
}

public class Dog implements Named {

    @AssertTrue private boolean isMale;

    public String getName() { ... };

}

The name property will be checked for nullity when the Dog bean is validated.