fix BigDecimal error,for issue alibaba#1831

core/src/main/java/com/alibaba/fastjson2/util/IOUtils.java

@@ -359,6 +359,7 @@ public static int writeDecimal(char[] buf, int off, long unscaledVal, int scale)
                 long power = POWER_TEN[scale - 1];
                 long div = unscaledVal / power;
                 long rem = unscaledVal - div * power;
+                int remSize = stringSize(rem);
                 off = IOUtils.writeInt64(buf, off, div);
                 buf[off] = '.';
                 if (scale == 1) {
@@ -373,6 +374,12 @@ public static int writeDecimal(char[] buf, int off, long unscaledVal, int scale)
                     buf[off + 2] = (char) (v >> 32);
                     buf[off + 3] = (char) (v >> 48);
                     return off + 4;
+                }else {
+                    int temp = remSize;
+                    while (temp < scale) {
+                        buf[++off] = '0';
+                        temp++;
+                    }
                 }
                 return IOUtils.writeInt64(buf, off + 1, rem);
             }

core/src/test/java/com/alibaba/fastjson2/issues_1800/Issue1831.java

@@ -0,0 +1,42 @@
+package com.alibaba.fastjson2.issues_1800;
+
+import com.alibaba.fastjson2.JSONObject;
+import lombok.Getter;
+import lombok.Setter;
+import lombok.ToString;
+import org.junit.jupiter.api.Test;
+
+import java.io.Serializable;
+import java.math.BigDecimal;
+
+import static org.junit.Assert.assertEquals;
+
+public class Issue1831 {
+
+    @Getter
+    @Setter
+    @ToString
+    public static class TestCls
+            implements Serializable {
+        private static final long serialVersionUID = 1L;
+        private BigDecimal goodsWeight;
+        private BigDecimal mineGrossWeight;
+        private BigDecimal mineTareWeight;
+        private BigDecimal mineWeighBridgeNet;
+        private BigDecimal mineNetBeforeSapmle;
+        private BigDecimal mineNetAfterSapmle;
+    }
+    @Test
+    public void Test() {
+        TestCls bean = new TestCls();
+        bean.setGoodsWeight(new BigDecimal("1.0009600000"));
+        bean.setMineGrossWeight(new BigDecimal("1.0"));
+        bean.setMineNetAfterSapmle(new BigDecimal("10.9600000"));
+        bean.setMineNetBeforeSapmle(new BigDecimal("1.096"));
+        bean.setMineWeighBridgeNet(new BigDecimal(".103"));
+        String expected = "{\"goodsWeight\":1.0009600000,\"mineGrossWeight\":1.0," +
+                "\"mineNetAfterSapmle\":10.9600000,\"mineNetBeforeSapmle\":1.096,\"" +
+                "mineWeighBridgeNet\":0.103}";
+        assertEquals(expected, JSONObject.toJSONString(bean));
+    }
+}