Java toString方法重写工具之ToStringBuilder案例详解

apache的commons-lang3的工具包里有一个ToStringBuilder类，这样在打日志的时候可以方便的打印出类实例中的各属性的值。

具体用法如下：

import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;

public class Message {

	private String from;

	private String to;

	private String body;

	public String getFrom() {
		return from;
	}

	public void setFrom(String from) {
		this.from = from;
	}

	public String getTo() {
		return to;
	}

	public void setTo(String to) {
		this.to = to;
	}

	public String getBody() {
		return body;
	}

	public void setBody(String body) {
		this.body = body;
	}

	@Override
	public String toString() {
		return ToStringBuilder.reflectionToString(this, ToStringStyle.MULTI_LINE_STYLE);
	}

	public static void main(String[] args) {
		Message msg = new Message();
		msg.setFrom("vince");
		msg.setTo("mike");
		msg.setBody("hello");
		System.out.println(msg.toString());
	}
}

而且支持多种打印格式

多行输出的：

com.vince.im.dto.Message@af72d8[
from=vince
to=mike
body=hello
]

默认一行的：

com.vince.im.dto.Message@af72d8[from=vince,to=mike,body=hello]

NO_FIELD_NAMES_STYLE：

com.vince.im.dto.Message@af72d8[vince,mike,hello]

SHORT_PREFIX_STYLE：

Message[from=vince,to=mike,body=hello]

SIMPLE_STYLE：

vince,mike,hello

原理其实就是通过JAVA的reflect（反射）获取值，然后组成一个Buffer。

里面部分源码：

    /**
     * <p>Append to the <code>toString</code> the start of data indicator.</p>
     * 拼装结果的
     * @param buffer  the <code>StringBuffer</code> to populate
     * @param object  the <code>Object</code> to build a <code>toString</code> for
     */
    public void appendStart(final StringBuffer buffer, final Object object) {
        if (object != null) {
            appendClassName(buffer, object);
            appendIdentityHashCode(buffer, object);
            appendContentStart(buffer);
            if (fieldSeparatorAtStart) {
                appendFieldSeparator(buffer);
            }
        }
    }

    /**
     * <p>Append the {@link System#identityHashCode(java.lang.Object)}.</p>
     * 拼装对象hashcode
     * @param buffer  the <code>StringBuffer</code> to populate
     * @param object  the <code>Object</code> whose id to output
     */
    protected void appendIdentityHashCode(final StringBuffer buffer, final Object object) {
        if (this.isUseIdentityHashCode() && object!=null) {
            register(object);
            buffer.append('@');
            buffer.append(Integer.toHexString(System.identityHashCode(object)));
        }
    }

需要注意的是：

Builds a toString value using the default ToStringStyle through reflection.

It uses AccessibleObject.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manager, if the permissions are not set up correctly. It is also not as efficient as testing explicitly.

Transient members will be not be included, as they are likely derived. Static fields will not be included. Superclass fields will be appended.

也就是说transient和static修饰的属性不能打印出来，但是父类的是可以打印出来的，使用的时候一定要注意了。

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