使用java反射获取字段信息

Question

我正在开发这个程序，它将打印与对象相关的所有信息，这些信息包括：

对于 x 中的每个切片（对象层次结构中的类）：

• 打印切片的类名。

• 对于该切片中的每个非静态字段：

  • 打印所有字段修饰符，后跟字段名称。
  • 打印字段值如下：
    • 如果字段为 null，则打印 null。
    • 如果字段是原始类型，则打印其值
    • 如果字段是字符串，则打印其值
    • 如果字段是数组：
      • 循环遍历数组中的元素
      • 打印索引
      • 递归打印数组项的详细信息
    • 否则，递归打印字段的详细信息

我已经使用下面的代码打印了几乎所有的信息：

public void print(Object obj) {
        Class cl = obj.getClass();
        while(cl != null) {
            System.out.println("Class Name: " + cl.getName());
            Field[] fields = cl.getFields();
            System.out.println("FIELDS: ");
            for(int i=0; i<fields.length; i++) {
                String modifier = Modifier.toString(fields[i].getModifiers());
                String name = fields[i].getName();
                System.out.print("Modifier: "+modifier+" Name: "+name+ "  ");
                try {
                    if(fields[i].get(cl) == null)
                        System.out.print("Value: NULL");
                } catch (        IllegalArgumentException | IllegalAccessException ex) {
                    Logger.getLogger(JCP.class.getName()).log(Level.SEVERE, null, ex);
                }
                if(fields[i].getType().isPrimitive())
                    try {
                        System.out.println("Value: " + fields[i].get(cl));
                } catch (        IllegalArgumentException | IllegalAccessException ex) {
                    Logger.getLogger(JCP.class.getName()).log(Level.SEVERE, null, ex);
                }
                if(fields[i].getType().isArray()) {
                    try {
                        Object[] object=(Object[])fields[i].get(obj);
                        System.out.print("Values: ");
                        for(int j=0;j<object.length;j++){
                            System.out.print(object[j].toString() + ",");
                        }   } catch (            IllegalArgumentException | IllegalAccessException ex) {
                        Logger.getLogger(JCP.class.getName()).log(Level.SEVERE, null, ex);
                    }
                }
            }
            System.out.println();
            cl = cl.getSuperclass();
        } 
    }

在以下课程上进行测试：

public class hello {
    private int privateField1;
    private long privateField2;
    public int publicField1;
    public int publicField2;

    public hello() {
    }


}

我得到以下输出：

Class Name: hello
FIELDS: 
Mar 10, 2014 2:54:36 PM JCP print
SEVERE: null
java.lang.IllegalArgumentException: Can not set int field hello.publicField1 to java.lang.Class
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:164)
Modifier: public Name: publicField1  Modifier: public Name: publicField2  
Class Name: java.lang.Object
FIELDS: 

    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:168)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:55)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.getInt(UnsafeIntegerFieldAccessorImpl.java:56)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.get(UnsafeIntegerFieldAccessorImpl.java:36)
    at java.lang.reflect.Field.get(Field.java:379)
    at JCP.print(JCP.java:211)
    at JCP.start(JCP.java:36)
    at Main.main(Main.java:11)

Mar 10, 2014 2:54:36 PM JCP print
SEVERE: null
java.lang.IllegalArgumentException: Can not set int field hello.publicField1 to java.lang.Class
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:164)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:168)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:55)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.getInt(UnsafeIntegerFieldAccessorImpl.java:56)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.get(UnsafeIntegerFieldAccessorImpl.java:36)
    at java.lang.reflect.Field.get(Field.java:379)
    at JCP.print(JCP.java:219)
    at JCP.start(JCP.java:36)
    at Main.main(Main.java:11)

Mar 10, 2014 2:54:36 PM JCP print
SEVERE: null
java.lang.IllegalArgumentException: Can not set int field hello.publicField2 to java.lang.Class
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:164)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:168)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:55)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.getInt(UnsafeIntegerFieldAccessorImpl.java:56)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.get(UnsafeIntegerFieldAccessorImpl.java:36)
    at java.lang.reflect.Field.get(Field.java:379)
    at JCP.print(JCP.java:211)
    at JCP.start(JCP.java:36)
    at Main.main(Main.java:11)

Mar 10, 2014 2:54:36 PM JCP print
SEVERE: null
java.lang.IllegalArgumentException: Can not set int field hello.publicField2 to java.lang.Class
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:164)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:168)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:55)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.getInt(UnsafeIntegerFieldAccessorImpl.java:56)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.get(UnsafeIntegerFieldAccessorImpl.java:36)
    at java.lang.reflect.Field.get(Field.java:379)
    at JCP.print(JCP.java:219)
    at JCP.start(JCP.java:36)
    at Main.main(Main.java:11)

主要是在“fields[i].get(cl)”中抛出所有异常，请帮助我找出获取字段值的问题所在。

Answer 1

你必须使用：

fields[i].get(obj)

代替：

fields[i].get(cl)

因为您试图获取类的这个特定实例的值，而不是类本身。

Answer 2

您在这部分的代码中几乎没有问题

if(fields[i].get(cl) == null) {
    System.out.print("Value: NULL");
}

if(fields[i].getType().isPrimitive()) {
    System.out.println("Value: " + fields[i].get(cl));
}

if(fields[i].getType().isArray()) {
    Object[] object=(Object[])fields[i].get(obj);
    System.out.print("Values: ");

    for(int j=0;j<object.length;j++){
        System.out.print(object[j].toString() + ",");
    }
}

拳头是你在类型实例而不是类上调用方法。

fields[i].get(obj) 应该是 fields[i].get(cl)，当您检查私有值时，您会重复这一点。你也习惯了很多调用。如果将结果分配给局部变量，则可以简化代码的可读性。

public void print(Object obj) throws IllegalArgumentException, IllegalAccessException {

        Class type = obj.getClass();

        while(type != null) {

            System.out.printf("Class Name: %s ", type.getName());
            System.out.println("FIELDS: ");

            for(Fiedl field : type.getFields()) {

                String modifier = Modifier.toString(field.getModifiers());

                String name = field.getName();

                System.out.printf("Modifiers: %s Name:%s\n",modifier,name);

                Object   fieldValue = field.get(obj);
                Class<?> filedType  = field.getType();
                String   printValue = null;

                if(fieldValue == null) {
                     printValue = "NULL";
                } else 
                if(fieldType.isArray()) {
                     printValue = Arrays.toString((Object[]) fieldValue);
                } else {
                     printValue = String.valueOf(fieldValue);                
                }

                System.out.printf("Value: %s",printValue);

            }
            System.out.println();
            type = type.getSuperclass();
        } 
    }