equals中的疑惑！equals函数是synchronized,会出现__equalsCalc != null的情况吗？

Question

 private java.lang.Object __equalsCalc = null;
    public synchronized boolean equals(java.lang.Object obj) {
        if (!(obj instanceof Ad)) return false;
        Ad other = (Ad) obj;
        if (obj == null) return false;
        if (this == obj) return true;
        if (__equalsCalc != null) {
            return (__equalsCalc == obj);
        }
        __equalsCalc = obj;
        boolean _equals;
        _equals = true && 
            this.adGroupId == other.getAdGroupId() &&
            ((this.adType==null && other.getAdType()==null) || 
             (this.adType!=null &&
              this.adType.equals(other.getAdType()))) &&
            ((this.destinationUrl==null && other.getDestinationUrl()==null) || 
             (this.destinationUrl!=null &&
              this.destinationUrl.equals(other.getDestinationUrl()))) &&
            this.disapproved == other.isDisapproved() &&
            ((this.displayUrl==null && other.getDisplayUrl()==null) || 
             (this.displayUrl!=null &&
              this.displayUrl.equals(other.getDisplayUrl()))) &&
            ((this.exemptionRequest==null && other.getExemptionRequest()==null) || 
             (this.exemptionRequest!=null &&
              this.exemptionRequest.equals(other.getExemptionRequest()))) &&
            this.id == other.getId() &&
            ((this.status==null && other.getStatus()==null) || 
             (this.status!=null &&
              this.status.equals(other.getStatus())));
        __equalsCalc = null;
        return _equals;
    }

Answer 1

会啊，__equalsCalc可被任何不请求该equals方法所属对象监视锁的线程在任何时刻修改

Answer 2

__equalsCalc只是被初始化为null，可以被修改，它的定义并没有出现在synchronized修饰的equals方法里

Answer 3

public synchronized boolean equals(java.lang.Object obj) { 
//对__equalsCalc加锁
synchronized (equalsCalc )
        if (!(obj instanceof Ad)) return false; 
        Ad other = (Ad) obj; 
        if (obj == null) return false; 
        if (this == obj) return true; 
      if (__equalsCalc != null) { 
            return (__equalsCalc == obj); 
        } 
        __equalsCalc = obj; 
        boolean _equals; 
        _equals = true && 
            this.adGroupId == other.getAdGroupId() && 
            ((this.adType==null && other.getAdType()==null) || 
            (this.adType!=null && 
              this.adType.equals(other.getAdType()))) && 
            ((this.destinationUrl==null && other.getDestinationUrl()==null) || 
            (this.destinationUrl!=null && 
              this.destinationUrl.equals(other.getDestinationUrl()))) && 
            this.disapproved == other.isDisapproved() && 
            ((this.displayUrl==null && other.getDisplayUrl()==null) || 
            (this.displayUrl!=null && 
              this.displayUrl.equals(other.getDisplayUrl()))) && 
            ((this.exemptionRequest==null && other.getExemptionRequest()==null) || 
            (this.exemptionRequest!=null && 
              this.exemptionRequest.equals(other.getExemptionRequest()))) && 
            this.id == other.getId() && 
            ((this.status==null && other.getStatus()==null) || 
            (this.status!=null && 
              this.status.equals(other.getStatus()))); 
        __equalsCalc = null; 
}
        return _equals; 
    }

Answer 4

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